tests: Fix QUnit failures due to pending ajax requests
- Changed tests to use mocked audio element, where applicable, to decouple behaviour.
- Changed tests to immediately get response from mock server and fixed Ajax callbacks to work with this.
- Set default server URL to the empty string.
Bug: T194620
Change-Id: Iaafd4e267c2ef6ca7b292991cfb2a5638305b9d7