Consider legacy Edge workaround in test/media/playhead_unit.js

[theodab]

In VideoWrapper.movePlayhead_, there is a workaround for a legacy Edge bug that causes the video wrapper to try to seek up to 11 times in a row until the seek works.
Technically, some of our playhead unit tests were triggering this workaround; however, since the workaround is based on setTimeout and the tests were synchronous, we simply ended the tests before the setTimeout resolved, and didn't notice.
We should probably adjust the tests to account for this workaround, instead of just technically avoiding it like we are now. This will improve the accuracy of the tests, and will also let us use jasmine.clock().tick() in those tests instead of jasmine.clock().setDate().

[joeyparrish]

First, I think we should evaluate whether or not the workaround is still necessary. It would be ideal to be able to remove the hack completely instead of adjusting the tests.