Add new flag to run-tests/test action: `-listTestsOnly`

Sometimes we want just to get list of tests that are going to be run and don't actually run them. This flag will simulate testing and return "success" for all tests as if they were run and passed.
Merge pull request #356 from facebook/nekto/add_list_tests_flag

Add new flag to run-tests/test action: `-listTestsOnly`