Updated acceptance tests to match new behavior in video captions.

Updated the tests to reflect recent changes to how the video player handles captions:

The default video doesn't have captions, so the CC button gets hidden.  The test cases now upload subtitles explicitly so the CC button shows up when we need it.  This has the nice side-effect of testing the current subtitle-upload workflow.

Reviewers: @jzoldak @valera-rozuvan