Fix test.
The event listener doesn't block the promise, so the asserts inside the
listener don't run without this change (it's trivial to check by
expanding the asserts in the test summary).
Differential Revision: https://phabricator.services.mozilla.com/D217200
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1908907
gecko-commit: ba4dc8ba4f47a10c94d5c738afd0099a53060774
gecko-reviewers: jjaschke