Maybe it would be better to use `SAMPLE_VIDEO` here? (And in that case, maybe rename it to `SAMPLE_OGG_VIDEO`?) If `valid_video_file()` ever changes its logic, then passing in a string to a nonexistent file would cause problems here. Might also be better to rename this method to `test_valid_video_file` since ideally `valid_video_file` will be used for more than Ogg files in the future.