Bug 829110 - add tests for navigator.mozId.request oncancel() callback; r=ferjm

This provides a marionette-testable way to prove recent changes to the native identity implementation.  This will be helpful in marketplace testing.

The main addition is a test for the persona `oncancel()` callback.

The tests have been re-written to exercise the newly-renamed `navigator.mozId` instead of `navigator.id`.