Noticed ```openat``` system call with ```O_WRONLY``` flag and ```sync_file_range``` and ```truncate``` on WAL file when using ```rocksdb::DB::OpenForReadOnly``` by way of ```db_bench --readonly=true --benchmarks=readseq --use_existing_db=1 --num=1 ...```
  I think introduced by facebook@7d7f144

This catches the WAL file being truncated and the modification time on it changing.
I am not sure if a mock filesystem with mock clock could be used to avoid having to sleep 1.1s.
The test could also check the set of files is the same and that the sizes are also unchanged.

Before:
 
[ RUN      ] DBBasicTest.ReadOnlyReopenMtimeUnchanged
db/db_basic_test.cc:182: Failure
Expected equality of these values:
  file_mtime_after_readonly_reopen
    Which is: 1621611136
  file_mtime_before_readonly_reopen
    Which is: 1621611135
  file is: 000010.log
[  FAILED  ] DBBasicTest.ReadOnlyReopenMtimeUnchanged (1108 ms)


After:
 
[ RUN      ] DBBasicTest.ReadOnlyReopenMtimeUnchanged
[       OK ] DBBasicTest.ReadOnlyReopenMtimeUnchanged (1108 ms)