Initially, every library greentea test has its project() creation in
their CMake. As running greentea using CTest move all greentea test
suite under one global project mbed-os and MBED_CONFIG_PATH set at
the root mbed os CMake under the condition BUILD_GREENTEA_TESTS
check so refactored rtos greentea CMake accordingly.
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.
Rajkumar Kanagaraj
Hari Limaye