Added test case for restacking relative to destroyed windows.
Under TDD methodology, this should be a failing test, however it isn't, because (LP: #1088399) is a race condition between when we receive MapRequest for a client and map the window and then later between that, destroy it and receive a CreateNotify for a new window on top of it, and restack the new window on top of the not-yet-mapped, but-also-destroyed window