Unit tests: Move platform target_h stubs

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.

platform/tests/UNITTESTS/doubles/CMakeLists.txt

@@ -4,6 +4,9 @@
 add_library(mbed-headers-platform INTERFACE)
 target_include_directories(mbed-headers-platform
     INTERFACE
+        .
+        platform
+        platform/cxxsupport
         ${mbed-os_SOURCE_DIR}/platform/include
         ${mbed-os_SOURCE_DIR}/platform/include/platform
         ${mbed-os_SOURCE_DIR}/platform/randlib/include/mbed-client-randlib/