CMake: unit-tests: Make platform stub only depend on headers it uses ...

Previously the platform stub library 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 actually using.

CMake: unit-tests: Remove trailing whitespace

CMake: Remove all unittest.cmake script from test suite ...

- Remove redundant cmake script as already added the CMake configuration file
- Remove redundant empty_baseline as it is no longer needed with the help of CMake configuration file

CMake: Refactor platform unittest CMake ...

- Add CMake configuration file test suites

Relocate platform unit tests