This test has a fission-specific failure right now, where the failure looks
like a case where the cross-origin resource simply hasn't loaded, or hasn't
loaded *yet*.
This patch adds a short delay (60 "executeSoon" ticks, each of which spin the
event loop). This doesn't make the test take appreciably longer on my machine,
but it does introduce a short delay. If it helps, it'll let us know that
there's simply a race condition which is causing this test's snapshots to be
taken before its iframes' resources are ready.
Differential Revision: https://phabricator.services.mozilla.com/D114285
