This tests the early return steps of "prepare a script" that come
_before_ step 10 "Set the element's "already started" flag". The
relevant steps are steps 6, 7 and 8. If this algorithm returns on any
of these steps, the script can be reinserted matching the requirements
and will run.
https://html.spec.whatwg.org/multipage/scripting.html#prepare-a-script
I wrote this test page up while testing something else, but found a bug
in Firefox where it doesn't allow re-preparing the script if step 8
fails: https://bugzilla.mozilla.org/show_bug.cgi?id=1735590
