async function runTest(callback) {
    async function onTick(currentTime) {
      if (callback) {
        await callback();
      }
      if (playing) {
        presentationTimeInSeconds++;
      }
    }
    // No test should require more than 60 seconds of simulated time.
    await Util.fakeEventLoop(60, onTick);
  }
Runs the fake event loop.
@param {function()=} callback An optional callback that is executed
  each time the clock ticks.