Bug 1297036: [MSE] P6. Fix invalid mochitest. r=gerald

The test accidentally worked because any demuxing failures in ended mode would be treated as EOS. There's no audio between [0-3), so playback couldn't start

MozReview-Commit-ID: 4B90CrVUTy4

--HG--
extra : rebase_source : 9079aa8a6983877745baac71c7868ca360b85095

dom/media/mediasource/test/test_TimestampOffset_mp4.html

@@ -71,9 +71,11 @@ runWithMSE(function(ms, el) {
       return Promise.all([audiosb.updating ? once(audiosb, 'updateend') : Promise.resolve(),
                           videosb.updating ? once(videosb, 'updateend') : Promise.resolve()]);
     }).then(function() {
-      ms.endOfStream();
+      info("waiting for play to complete");
-      once(el, 'ended').then(SimpleTest.finish.bind(SimpleTest));
       el.play();
+      el.currentTime = el.buffered.start(0);
+      ms.endOfStream();
+      Promise.all([once(el, 'ended'), once(el, 'seeked')]).then(SimpleTest.finish.bind(SimpleTest));
     });
   });
 });