Bug 1451221 - Don't run test_background_video_suspend_ends.html in parallel. r=cpearce

When this test times out, one of the two videos in parallel takes
minutes to start.

MozReview-Commit-ID: AhXsQKDKwOD

--HG--
extra : rebase_source : 3d42329967204c201b9e882b13dafc3149247c79

dom/media/test/test_background_video_suspend_ends.html

@@ -8,6 +8,8 @@
 <script type="text/javascript">
   "use strict";
 
+  PARALLEL_TESTS = 1;
+
   var manager = new MediaTestManager;
 
   async function runTest(test, token) {