Replace QUnit.push() with assert.push() or this.push()

[platinumazure]

As noted in the QUnit 2.x Upgrade Guide, the method for pushing custom assertions has moved from QUnit to the assert object.

This one might be trickier because we may need to differentiate between whether we are in an assertion function (use this.push) or in a custom test/handler (perhaps use QUnit.config.current somehow?). This is probably lower priority as well.