Bug 1665941 - add WPT for requestFullscreen in a tree of cross-origin iframes. r=smaug DONTBUILD

In order to more thoroughly test fullscreen with Fission, this change
introduces a new WPT with a more complicated tree of cross-origin
iframes. This is a manual test since WPT can't fake user interaction
in a cross-origin iframe yet. Once this is supported it should be
possible to call requestFullscreen() in automation and convert
this test.

Differential Revision: https://phabricator.services.mozilla.com/D90728