Unit tests: Libraries should not contain test-specific logic

[LDong-Arm]

Description of defect

(Raised from #14870 (comment))

A few libraries check UNITTEST:

$ grep -rn '\bUNITTEST\b'
drivers/include/drivers/MbedCRC.h:32:#ifdef UNITTEST
UNITTESTS/target_h/platform/mbed_assert.h:44:#ifndef UNITTEST
storage/kvstore/tdbstore/tests/UNITTESTS/TDBStore/CMakeLists.txt:10:        UNITTEST
storage/kvstore/filesystemstore/tests/UNITTESTS/FileSystemStore/CMakeLists.txt:10:        UNITTEST
rtos/include/rtos/internal/mbed_rtos_storage.h:20:#if MBED_CONF_RTOS_PRESENT || defined(UNITTEST)
rtos/include/rtos/internal/mbed_rtos1_types.h:20:#if MBED_CONF_RTOS_PRESENT || defined(UNITTEST)
rtos/include/rtos/Thread.h:36:#if MBED_CONF_RTOS_PRESENT || defined(DOXYGEN_ONLY) || defined(UNITTEST)
rtos/include/rtos/mbed_rtos_types.h:20:#if MBED_CONF_RTOS_PRESENT || defined(DOXYGEN_ONLY) || defined(UNITTEST)

But production code should not contain test-specific checks. Instead, fakes/mocks/stubs/... should be enough to wrap any tested modules without modifying interfaces for production uses.

Target(s) affected by this defect ?

N/A

Toolchain(s) (name and version) displaying this defect ?

N/A

What version of Mbed-os are you using (tag or sha) ?

mbed-os-6.12.0

What version(s) of tools are you using. List all that apply (E.g. mbed-cli)

N/A

How is this defect reproduced ?

N/A