The test suite runs in a shared window - It needs to set everything back to the way it was found...

An example: https://github.com/mozilla-b2g/gaia/blob/master/apps/clock/test/unit/picker/spinner_test.js#L13-L24 and https://github.com/mozilla-b2g/gaia/blob/master/apps/clock/test/unit/picker/spinner_test.js#L43-L45