Local copy of Xcode (Lion)
I believe your best option is to use SuperDuper! to make a backup image of each machine with a clean instal of the OS and any software you know will be needed for testing every time. Once you have created the backup image it will make reinstalls much faster and easier.

I have not tried copying the Xcode.app before, but I think you would be missing a lot of preferences and command line tools if all you copied was the app.

EDIT:
If you download Xcode from the App Store and copy that Xcode.app package to another machine without ever launching it, it should work fine. There should be no preference files or app support until after the first time Xcode is launched. From my point of view though, this is not a very useful way of putting test/development machines together though because when I use Xcode I have it set-up a certain way with command line tools installed which would need to be done on each machine when you copy the Xcode.app.