You might be thinking of this:
http://forum.shrapnelgames.com/showthread.php?t=45908
but I've never used it and it doesn't appear to be available anymore. I use the battle sim map located here:
http://www.dom3minions.com/utilities.htm
I just edit the map file to include the units I want to test and start a game with the debug mod to get access to spells.
Debug mod:
http://forum.shrapnelgames.com/showthread.php?t=36453