DoSetAndLoadFavicon can return early without doing any work, and there's no way to know that it did so (e.g. for the tests in bug 526920).

actually in bug 553489 i wrote a test that just sets invalid icons, and finally a valid icon, with some spacing in the middle, since this uses async statements they will be enqueued. This should be enough for tests, i thought about implementing cancel and fail callbacks, but either they were making the interface uglier or adding useless calls to implementers. I'd say wontfix unless someone has anything against that.