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
(cherry picked from commit 0cd9bc4755307186d2b05e11d93c463d532d604f)

Conflicts:
	apps/sms/js/message_manager.js
	apps/sms/js/sms_mock.js
	apps/sms/js/thread_ui.js
	apps/sms/test/unit/sms_test.js