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.