add test_results verb

@wjwwood @tfoote Please review.
It currently matches exactly the proven code in https://github.com/ament/ament_tools/blob/7642e3ea8e12dd2a98a69b5f02b6f01a235ccadf/ament_tools/helper.py#L42
Without extensive testing I wouldn't feel confident in refactoring it and don't see a need to spend any time on it.