CMake: unit-tests: Make drivers UNITTESTS only depend on headers it uses

Previously the drivers unit tests depended on `mbed-headers`, which
is a collection of all available headers in mbed-os. To make it easier
to separate the library, only depend on the headers we're using.

Author: rajkan01

drivers/tests/UNITTESTS/PwmOut/CMakeLists.txt

@@ -18,7 +18,10 @@ target_sources(${TEST_NAME}
 
 target_link_libraries(${TEST_NAME}
     PRIVATE
-        mbed-headers
+        mbed-headers-base
+        mbed-headers-platform
+        mbed-headers-hal
+        mbed-headers-drivers
         mbed-stubs-hal
         mbed-stubs-platform
         gmock_main

drivers/tests/UNITTESTS/Watchdog/CMakeLists.txt

@@ -19,7 +19,10 @@ target_sources(${TEST_NAME}
 
 target_link_libraries(${TEST_NAME}
     PRIVATE
-        mbed-headers
+        mbed-headers-base
+        mbed-headers-hal
+        mbed-headers-drivers
+        mbed-headers-platform
         mbed-stubs-platform
         mbed-stubs-hal
         gmock_main