Bug 903260 - [SMS][MMS] SettingsURL value is never used in sms/js/notification.js: Ringtones BROKEN

https://bugzilla.mozilla.org/show_bug.cgi?id=903260

- Make the mock's set method behave enough like the real thing to write a correct test
- Assign the result of the call to set(…) to the actual local setting
- Tests
    - Ensure that the src property is as expected

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>