Bug 1548446 - part2 : should wait until seek completed. r=jolin

Main test function should await `seekVideo()`  until it finishes.

Differential Revision: https://phabricator.services.mozilla.com/D30954

--HG--
extra : moz-landing-system : lando

dom/media/test/test_webvtt_seeking.html

@@ -23,16 +23,16 @@ async function startTest() {
   const cues = createCues(video);
   await startVideo(video);
 
-  seekVideo(video, cues[0].startTime);
+  await seekVideo(video, cues[0].startTime);
   await waitUntilCueIsShowing(cues[0]);
   checkActiveCueAndInactiveCue(cues[0], cues[1]);
 
-  seekVideo(video, cues[1].startTime);
+  await seekVideo(video, cues[1].startTime);
   await waitUntilCueIsShowing(cues[1]);
   checkActiveCueAndInactiveCue(cues[1], cues[0]);
 
   // seek forward again
-  seekVideo(video, cues[0].startTime);
+  await seekVideo(video, cues[0].startTime);
   await waitUntilCueIsShowing(cues[0]);
   checkActiveCueAndInactiveCue(cues[0], cues[1]);