Changed test to comply with spec.
Due to changes before automation was introduced this
wpt test does not adhere to the spec.
In this test the iframe enters 2 elements into it's toplayer
making it a non-simple fullscreen doc. But the old test
expects it to exit fullscreen for itself and document
by only making 1 call to exitFullscreen - it also erroneously
expect (fullscreenchange, element) to be seen on document for
that first call, which it should not see.
I've also added to the test, what the old test expects
(1 call to exitFullscreen exits everything) but that call
is now on document.exitFullscreen() as that, per the spec
should exit all descendantDocs out of fullscreen,
whether or not they are simple fullscreen elements.
Comments in the test explain what's going on.
Differential Revision: https://phabricator.services.mozilla.com/D219853
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1914336
gecko-commit: c4d3a3de444e3a1b7c3365b7ccb37172f08a5e45
gecko-reviewers: zcorpan