Refactor CEHTML tests prior to adding new ones

Pull in getToPlaying() helper function from HTML5 tests. Swap to using it where
possible.
Add fireSentinels() helper from HTML5 tests. Swap to using it.
Introduce configureSeekToFail() helper, change tests to use it
Add test to ensure pause sentinel retries twice for CEHTML
Small refactor on eventTypeHasFired() for clarity
Pause sentinel should give up and emit a failure event on the third time it has to attempt to pause
Ensure calling resume then pause resets the pause sentinel attempt count

Added TODO around whether calling pause() from the PAUSED state should or should not reset the pause sentinel attempt count
Ensure seek sentinel is retried twice
CEHTML seek sentinel gives up on the third attempt