Fix the popover-events.html failure in wpt.fyi.

At D176675, we replaced the call of waitForRender() with waitForTick() due to Intermittent result for the test(Bug 1828826). However, wpt.fyi result (which is now available after we enabled nightly) shows that the test fails.

Reverting waitForTick() to waitForRender() seems fix the issue at wpt.fyi. A follow-up patch might be needed to update .ini expectation file if there is intermittent compalins from Gecko run.

Differential Revision: https://phabricator.services.mozilla.com/D195091

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1867486 gecko-commit: e1af579f3385dd2aac37a6f13fa461de05c14f3f gecko-reviewers: emilio