Bug 1042478: AirplaneModeHelper.ready is invoked several times.

And in order to make `mozSettings.mTeardown()` can influence the other test cases to make them more clean, please move this function to https://github.com/PinZhang/gaia/blob/pzhang/bug1042478-airplanemodehelper/apps/settings/test/unit/airplane_mode_helper_test.js#L22 here.