Assumption that greentea test file is always named main.cpp is
incorrect. Updated mbed_greentea_add_test() macro to make TEST_SOURCES
parameter compulsory, which is used to specify greentea test
file(s). This allows tests to use C, or have a different name.
Therefore also updated all pre-existing greentea test CMake files to
explicity add main.cpp to TEST_SOURCES.
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.
Jaeden Amero
Hari Limaye
Meano
jeromecoutant
Rajkumar Kanagaraj
Ashok Rao