Test by Gijs. Note that I removed the .isActive assertion, because this doesn't
affect activeness.
We could make that change too, but that's a bigger change, and I think we want
to do this regardless.
The CAN_RUN_SCRIPT annotations are obsolete (they come from bug 1582042
which directly fired docshell activity events, but that's not true
anymore, bug 1635914 changed the setup a bit later).
Differential Revision: https://phabricator.services.mozilla.com/D185643
93252b3465