Convert video-source* tests to testharness.js

Cleaning up video-source* tests in media/
to use testharness.js instead of video-test.js.
This will enable to upstream these tests to web-platform-tests.

BUG=588956

Review-Url: https://codereview.chromium.org/2102433003
Cr-Commit-Position: refs/heads/master@{#402149}
6 files changed