CMake: unit-tests: Split platform headers out of mbed-stubs-headers

We have a central collection of "stub headers", which makes reasoning
about dependencies rather difficult, as it forces every stub library to
depend on all available stub headers. The standard approach would be for
each stub library to expose its public headers, and its dependents to
explicitly specify a dependency on the stub library containing the
headers it needs. This is a more modular design than creating a
header-only monolith library. Move the platform stub headers from this
central library into the mbed-stubs-platform library to increase
modularity.

mbed-stubs-connectivity now depends on the mbed-stubs-platform because
it requires some headers which were moved to mbed-stubs-platform.

Author: rwalton-arm

UNITTESTS/stubs/connectivity/CMakeLists.txt

@@ -62,5 +62,6 @@ target_link_libraries(mbed-stubs-connectivity
         mbed-headers
         mbed-stubs-headers
         mbed-stubs-rtos
+        mbed-stubs-platform
         gtest
 )

platform/tests/UNITTESTS/doubles/CMakeLists.txt

@@ -26,6 +26,10 @@ target_sources(mbed-stubs-platform
         randLIB_stub.c
         randLIB_stub.cpp
 )
+target_include_directories(mbed-stubs-platform
+    PUBLIC
+        .
+)
 target_link_options(mbed-stubs-platform
     PRIVATE
         --coverage
@@ -35,5 +39,4 @@ target_link_libraries(mbed-stubs-platform
         mbed-headers-base
         mbed-headers-hal
         mbed-headers-platform
-        mbed-stubs-headers
 )