Use add_subdirectory to include individual test modules and other scope related fix
Created by: girishpathak
This PR changes unit_test/CMakeLists.txt to include individual test modules using CMake's 'add_subdirectory' command instead of 'include' to keep the scope of the test module from the parent. It also fixes minor FWK_SRC scoping, see commit module_common: fix FWK_SRC scoping for details