Bug 968619 - [Settings] add marionette test for airplane mode

The some of below code is just duplicated of the code in `root.js`[1]. And we should just add the selectors of `AirplaneMode.Selectors` into `root.js`, because these selectors are just in the root panel.
[1] https://github.com/mozilla-b2g/gaia/blob/master/apps/settings/test/marionette/app/regions/root.js