Adds the ability for the Youtube extractor to fetch the selected game on a video.
The first regex is for games which have an image, the second for games which don't.
The example in #10813 requires TV provider authentication in Firefox,
while youtube-dl can download it directly with an US proxy.
I'm not sure whether the mgid fix is cmt-specific or it applies to all
mtv-based sites. I keep it in cmt.py until similar patterns are found in
other websites.
I change the URL of the third test case, because now the original URL
does not contain a video anymore, and there's no easy to get the real
URL from the /film/ one.