Rely on mediawiki_selenium's screenshot on failure

This is I123a820a278b52e889f5a3dece6f4e55ad4a4c85 in the
mediawiki/selenium repository.  We can merge this ahead of time but we'll
lose the screenshots.

Change-Id: Iaf446a43f4662db71f13c92a98c42fdfb2413f83