test: repeat_unit_test.sh don't core dump to stdout

[jaki]

When using --num_repetitions of yb_build.sh for running tests multiple times, it produces a nice, compact output of fail/pass to stdout:

PASSED: iteration 18, 125 sec                                                                                                                                                                                                                                
PASSED: iteration 19, 125 sec                                                                                                                                                                                                                                
[2021-01-26T14:41:54 common-test-env.sh:1150 did_test_succeed] Test failure reason: exit code: 1                                                                                                                                                             
FAILED: iteration 20, 135 sec; test log path: /nfusr/dev-server/jason/logs/repeat_unit_test/pg_index_backfill-test/PgIndexBackfillTest.CreateUniqueIndexWriteAfterSafeTime/2021-01-26T13_57_54/20.log.gz                                                     
PASSED: iteration 21, 133 sec                                                                                                                                                                                                                                
[2021-01-26T14:46:24 common-test-env.sh:1150 did_test_succeed] Test failure reason: exit code: 1                                                                                                                                                             
FAILED: iteration 22, 135 sec; test log path: /nfusr/dev-server/jason/logs/repeat_unit_test/pg_index_backfill-test/PgIndexBackfillTest.CreateUniqueIndexWriteAfterSafeTime/2021-01-26T13_57_54/22.log.gz                                                     
PASSED: iteration 23, 122 sec                                                                                                                                                                                                                                
PASSED: iteration 24, 136 sec

Recently, I saw a core dump that ended up getting spit out:

PASSED: iteration 5, 228 sec                                                                                                                                                                                                                                 
PASSED: iteration 6, 221 sec                                                                                                                                                                                                                                 
[2021-01-26T16:45:44 common-test-env.sh:1150 did_test_succeed] Test failure reason: exit code: 1                                                                                                                                                             
Found a core file at '/tmp/yb_tests__2021-01-26T16_35_41__11657.32444.28901/core.23188', backtrace:                                                                                                                                                          
+ echo ''                                                                                                                                                                                                                                                    
+ gdb -q -n -ex bt -ex 'thread apply all bt' -batch /nfusr/dev-server/jason/code/yugabyte-db/2/build/asan-clang-dynamic-ninja/tests-pgwrapper/pg_index_backfill-test /tmp/yb_tests__2021-01-26T16_35_41__11657.32444.28901/core.23188                                                     
+ grep -Ev '^\[New LWP [0-9]+\]$'                                                                                             
+ /nfusr/dev-server/jason/code/yugabyte-db/2/build-support/dedup_thread_stacks.py                                             
+ tee -a /nfusr/dev-server/jason/logs/repeat_unit_test/pg_index_backfill-test/PgIndexBackfillTest.MasterLeaderStepdown/2021-01-26T16_18_18/7.log                                                                                                                                         
                                                                                                                                                              
warning: core file may not match specified executable file.                                                                   
                                                                                                                                            
warning: File "/opt/yb-build/brew/linuxbrew-20181203T161736v9-3ba4c2ed9b0587040949a4a9a95b576f520bae/Cellar/glibc/2.23/lib/libthread_db-1.0.so" auto-loading has been declined by your `auto-load safe-path' set to "$debugdir:$datadir/auto-load:/usr/bin/mono-gdb.py".                  
To enable execution of this file add                                                                                                                          
        add-auto-load-safe-path /opt/yb-build/brew/linuxbrew-20181203T161736v9-3ba4c2ed9b0587040949a4a9a95b576f520bae/Cellar/glibc/2.23/lib/libthread_db-1.0.so                                                                                                                           
line to your configuration file "/nfusr/dev-server/jason/.gdbinit".                                                                                                                                                                                          
To completely disable this security protection add                                                                            
        set auto-load safe-path /                                                                                                                             
line to your configuration file "/nfusr/dev-server/jason/.gdbinit".                                                           
For more information about this security protection see the                                                                   
"Auto-loading safe path" section in the GDB manual.  E.g., run from the shell:                                                                                
        info "(gdb)Auto-loading safe path"                                                                                                  
                                                                                                                              
warning: Unable to find libthread_db matching inferior's thread library, thread debugging will not be available.                            
                                                                                                                              
warning: File "/opt/yb-build/brew/linuxbrew-20181203T161736v9-3ba4c2ed9b0587040949a4a9a95b576f520bae/Cellar/glibc/2.23/lib/libthread_db-1.0.so" auto-loading has been declined by your `auto-load safe-path' set to "$debugdir:$datadir/auto-load:/usr/bin/mono-gdb.py".                  
                                                                                                                                                                                                                                                             
warning: Unable to find libthread_db matching inferior's thread library, thread debugging will not be available.              
Core was generated by `/nfusr/dev-server/jason/code/yugabyte-db/2/build/asan-clang-dynamic-ninja/tests'.                                    
Program terminated with signal 3, Quit.                                                                                       
#0  0x00007ff51e1c146d in nanosleep () at ../sysdeps/unix/syscall-template.S:84                                 
84      ../sysdeps/unix/syscall-template.S: No such file or directory.                                                                      
#0  0x00007ff51e1c146d in nanosleep () at ../sysdeps/unix/syscall-template.S:84                                                                               
#1  0x00007ff51ebe904b in std::__1::this_thread::sleep_for(std::__1::chrono::duration<long long, std::__1::ratio<1l, 1000000000l> > const&) () from /opt/yb-build/thirdparty/yugabyte-db-thirdparty-v20201207195011-bbf28cf4f1-centos7-linuxbrew/installed/asan/libcxx/lib/libc++.so.1    
#2  0x00007ff545baf88f in std::__1::this_thread::sleep_for<long long, std::__1::ratio<1l, 1000l> > (__d=...) at /opt/yb-build/thirdparty/yugabyte-db-thirdparty-v20201207195011-bbf28cf4f1-centos7-linuxbrew/installed/asan/libcxx/include/c++/v1/thread:448

This core dump ends up in the log file, so there's no benefit of printing to stdout. It ruins the nice compact summary of pass/fail.