#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.
Dictionaries with more than one key should be spread across multiple lines:
```python
data = {
    'hello': 'world',
    'foo': 'bar',
}
```

Same for lists with more than one element:
```python
data = [
    'foo',
    'bar',
]
```