Bug 1047168 - Modify test_play_youtube to play a video file locally

The player.is_fullscreen function already checks that the document is in full screen.
I can try and do an extra check that the video element is the mozFullScreenElement, but last time I tried that, I failed.

When we are in full screen and the script taps on the full screen button again, it's basically checking that the fullscreen button is present, otherwise it would fail.
I could add some checks videcontrols.getBoundingClientRect.top, etc, returns something else than 0.