Bug 1071706 - Dark icons on lockscreen r=mhenretty

Actually I'd like to rework this a bit:
* a `lockscreen support` suite
* a `setup` sending the `lockscreen-appopened` with a separate `lockscreenApp` object
* a `teardown` making sure we always finish with a `lockscreen-appclosing` (to make sure we don't have any side effects on other tests)
* one test asserting that the classes are properly set for the `lockscreenApp` object
* one test calling `setAppearance` with a different `app` object and making sure nothing changed (pretty much what we have right now)
* one test sending `lockscreen-appclosing` and making sure the StatusBar is correctly setup for the current active app