Update QUnit tests to use sinon.js from core

Change I789fb71903f26dbbfe781f010dc5cd3756759943 in mediawiki/core
introduced sinon.js which conflicted with sinon.js in MobileFrontend.

This removes sinon.js from MobileFrontend and converts QUnit tests to
use the one included in core instead.

Sandbox takes care of restoring the original functions so many
teardowns could be removed.

Change-Id: I8ad1da1ab7dd09050e6fcde7a3f58506d4a5e22d