Introduce helper that waits for Vue events in tests

From https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28207#note_167937643:

It would be nice to have:

-  const eventTriggered = new Promise(resolve => eventHub.$once('someEvent', resolve));
+  const eventTriggered = waitForVueEvent(eventHub, 'someEvent');

Maybe this could even be an upstream contribution to https://vue-test-utils.vuejs.org/.