Bug 863776 - Fix SMS tests caused by Bug 843511 r=borja

* rename the desktop mock to not conflict with the unit test mock
* better cleanup in the sms_test 'unit' test
* moved some of the `ThreadUI` tests that were in `sms_test` to `thread_ui_test`
* stubbed window.history.back in sms_test.js so that we don't leave the
  test-agent