Bok-Choy CMS Video Tests

Bok-Choy tests for [video.feature](https://github.com/edx/edx-platform/blob/master/cms/djangoapps/contentstore/features/video.feature)

@auraz @polesye @jzoldak 
I'd suggest rewrite it so:
```python
if vertical:
  return '{vertical} {video_element}'.format(
    vertical=self.get_video_vertical_selector(video_display_name),
    video_element=class_name),
  )
else:
  return class_name
```