[Web Audio] Added Audio API tests: MediaElementAudioSource interface

* Test with source output to OfflineAudioContext
* Test with source output to ScriptProcessor
* Added trimEmptyElements() to helpers.js
Clean up MediaElementAudioSource test

Remove the debugging output, unused tests and clean up some of the
whitespace.
Remove MediaElementAudioSource OfflineAudioContext test

MediaElementAudioSource doesn't make sense in the context of an
OfflineAudioContext, so we'll remove that version of the test.
Tests for MediaElementAudioSource

Merging the changes from #396 with some cleaning up and code review.