#562 WIP - Refactoring YouTube tests as suggested in #517.

- Comparing expected sample video metadata to metadata retrieved from video file.
- Refactored valid_video_file() test cases into using one method with a list of valid/invalid cases.
I see why this is needed, but it seems like a hack. This might be a good time to look into pytest's [parametrized tests](http://pytest.org/latest/parametrize.html).