Stubs previously in UNITTESTS/target_h/ have the same names as
regular Mbed OS headers, intending to override the latter directly.
We move platform target_h stubs into
platform/tests/UNITTESTS/doubles/platform/.

Note: nvic_wrapper.h is normally implemented and used by Mbed targets
as needed. But as unit tests do not have a real target, we treat it
as a stub for the platform.