CMake: Refactor platform unittest CMake ...

- Add CMake configuration file test suites

[STD-PIN] BUTTON and LED define ...

hal/tests/TESTS/mbed_hal/gpio/main.cpp is replaced by
hal/tests/TESTS/pin_names/generic/main.cpp now

Fix CMake include path for the minimal-printf test

CMake: rename greentea test macro

CMake: Add platform greentea test with CMake

Merge pull request #13548 from evva-sfw/enhance-minimal-printf ...

Add width modifier and prepending zeros for hexadecimal output and decimal precision for floating point

Add width modifier and prepending zeros for integer, hexadecimal and floating point output as well as decimal precision for floating point

Change MBED_STATIC_ASSERTs version for built-in

Merge pull request #13563 from paul-szczepanek-arm/circ-buf ...

Add mutiple push and pop for circular buffer

Move greentea test closure to library

missing dependencies

unit tests for circular buffer

Move mbed_functional tests into platform/tests/TESTS

Adding required host_tests

Relocate platform unit tests

Refactoring \platform as per directory structure proposal. ...

Mbed OS will soon be changing directory structure to the below:

```
[component name]
├── mbed_lib.json
├── CMakeList.txt
├── README.md
├── include
│   └── [component name]
│       └── internal
├── source
└── tests
    ├── <framework>
    │   └── <test suite one>
    ├── UNITTESTS
    │   └── <unit test suite one>
    └── TESTS
        └── <greentea test suite one>
```
This PR implements the above new directory structure for the current \platform directory.
**NOTE:** This PR does NOT include the refactoring of UNITTESTS as shown in the above directory structure. This will be done in a subsequent PR.