qt_add_test_executable() for making simple tests

The test project wizard in Qt Creator is fine, but perhaps something like qt_add_test_executable() could also be implemented for CMake to make it easier to add simple tests to a project that don't need any special setup.

For example it could generate minimal main.cpp for the test and automatically set QUICK_TEST_SOURCE_DIR macro, and additionally do a call to add_test() for ctest support.