It was flaky since long ago, but has been consistently timing out since r207566
That commit seems totally unrelated to the underlying issue :)
The problem is that the page is muted by default, so the player mutes itself and that means the page state doesn't contain the IsPlayingAudio flag and because the video is set to loop, the test never ends.
Created attachment 331039 [details]
Committed r226769: <https://trac.webkit.org/changeset/226769>