Video: tests to check speed change

In this PR are added 2 tests to check functionality that fix [BLD-424](https://edx-wiki.atlassian.net/browse/BLD-424) .

1st test: js unit test.
2nd test: acceptance test. Please notice the way of testing current issue. Corner cases: depends on connection and video bufferization.

@wedaly , @jzoldak please review.