tests/test_tools: fails on iotlab-m3 due to core/cpu regression

[miri64]

Description

The test tests/test_tools fails on iotlab-m3. According to git bisect this is a regression introduced in 0ff9e55.

Steps to reproduce the issue

iotlab-experiment submit -d 10 -n RIOT-test_tools -l 1,archi=m3:at86rf231+site=grenoble
IOTLAB_NODE=auto-ssh BOARD=iotlab-m3 make -C tests/test_tools/ flash test

Expected results

Test succeeds (presented result from 0ff9e55~1

shellping
ssh -t lenders@grenoble.iot-lab.info 'socat - tcp:m3-101.grenoble.iot-lab.info:20000'
shellpong
true this should not be echoed
shellping
shellpong
toupper lowercase
LOWERCASE
getchar

getchar 0x0a

Actual results

Test fails on toupper test case

shellping
ssh -t lenders@grenoble.iot-lab.info 'socat - tcp:m3-101.grenoble.iot-lab.info:20000'
shellping
shellpong
shellpong
true this should not be echoed
shellping
toupper lowercase
shellpong

Traceback (most recent call last):
  File "/home/mlenders/Repositories/RIOT-OS/RIOT-release/tests/test_tools/tests/01-run.py", line 83, in <module>
    sys.exit(run(testfunc))
  File "/home/mlenders/Repositories/RIOT-OS/RIOT-release/dist/pythonlibs/testrunner/__init__.py", line 30, in run
    testfunc(child)
  File "/home/mlenders/Repositories/RIOT-OS/RIOT-release/tests/test_tools/tests/01-run.py", line 76, in testfunc
    _test_clean_output(child)
  File "/home/mlenders/Repositories/RIOT-OS/RIOT-release/tests/test_tools/tests/01-run.py", line 55, in _test_clean_output
    assert retline.strip() == 'LOWERCASE'
AssertionError
make: *** [/home/mlenders/Repositories/RIOT-OS/RIOT-release/tests/test_tools/../../Makefile.include:768: test] Error 1

Versions

Every version after 0ff9e55 until current master

Operating System Environment
----------------------------
         Operating System: "Arch Linux" 
                   Kernel: Linux 5.7.7-arch1-1 x86_64 unknown
             System shell: GNU bash, version 5.0.17(1)-release (x86_64-pc-linux-gnu)
             make's shell: GNU bash, version 5.0.17(1)-release (x86_64-pc-linux-gnu)

Installed compiler toolchains
-----------------------------
               native gcc: gcc (GCC) 10.1.0
        arm-none-eabi-gcc: arm-none-eabi-gcc (Arch Repository) 10.1.0
                  avr-gcc: missing
         mips-mti-elf-gcc: missing
               msp430-gcc: missing
     riscv-none-embed-gcc: missing
     xtensa-esp32-elf-gcc: missing
   xtensa-esp8266-elf-gcc: missing
                    clang: clang version 10.0.0 

Installed compiler libs
-----------------------
     arm-none-eabi-newlib: "3.3.0"
      mips-mti-elf-newlib: missing
  riscv-none-embed-newlib: missing
  xtensa-esp32-elf-newlib: missing
xtensa-esp8266-elf-newlib: missing
                 avr-libc: missing (missing)

Installed development tools
---------------------------
                   ccache: missing
                    cmake: cmake version 3.17.3
                 cppcheck: missing
                  doxygen: 1.8.18
                      git: git version 2.27.0
                     make: GNU Make 4.3
                  openocd: Open On-Chip Debugger 0.10.0+dev-01089-g3bfe49266 (2020-02-26-14:18)
                   python: Python 3.8.3
                  python2: Python 2.7.18
                  python3: Python 3.8.3
                   flake8: 3.8.3 (mccabe: 0.6.1, pycodestyle: 2.6.0, pyflakes: 2.2.0) CPython 3.8.3 on
               coccinelle: missing