It would be nice to avoid a four second delay while running this unit test. Ideally, a unit test would run in less than 100 ms. One way to achieve this would be to introduce a variable in forward_rewind_controller.js to hold the interval time used in startFastSeeking. The unit test can set this variable to a small value. See  https://github.com/mozilla-b2g/gaia/blob/master/apps/video/js/video.js#L900-L901 for a similar usage.