Production code should not contain any test-specific checks. Rather
than checking `UNITTEST`, unit tests can directly set
`MBED_CONF_RTOS_PRESENT=1` to make RTOS available for testing.
Note: The cellular ATHandler test also has `MBED_CONF_RTOS_PRESENT=1`
added because `ATHandler.cpp` contains a check of this variable.
Move the header-only mbed-headers-rtos library the unit test stubs
depend on into the rtos component directory. This makes the rtos
stubs more self-contained and improves the composition of the library.
Move rtos stubs headers to the mbed-os/rtos/tests/UNITTESTS/double
directory and update CMake to create a mbed-stubs-rtos-headers
library for rtos stub headers
Lingkai Dong
Rajkumar Kanagaraj