Convert text-track-selection* tests to testharness.js

Cleaning up text-track-selection* tests in media/track
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/2123003003
Cr-Commit-Position: refs/heads/master@{#403932}
4 files changed