Bug 1100390:  Create a new test case for the deletion of video from Preview

...kes video, plays it back, then deletes it in preview mode
We already use this wait in <code>record_video</code> [method][link]. We can remove. 
[link]:https://github.com/mozilla-b2g/gaia/blob/master/tests/python/gaia-ui-tests/gaiatest/apps/camera/app.py#L78