Bug 1073157 - [System] fix wallpaper_manager.js tests with a newer mocha...

... r=etienne
I didn't know you could pass a function to done()...

I'm guessing that the problem you're fixing here is that if any of the tests fail, done never gets called and we timeout instead of giving the correct message, right? If I had noticed that I would have used a try/catch to call done with the exception.  But if that is what passing this function to done() does, then that seems great.