Small comment fix
LoadingMessageSourceProviderTest: factorize "delayed answer" code

We will need it one more time...
LoadingMessageSourceProviderTest: add failing test for CancellationException

If a task times out, this can happen:

        thread1            thread2
        --------           --------
        grabs task
                           grabs task
                           get()s
        get()s
        timeout, cancels
                           BOOM: CancellationException