Well, for a script that should just use a mailbox if it's too large, I'd create a new user. First I create a mailbox that isn't large enough to trigger a move and run the script. If it doesn't do anything, it passes the test. Else it fails. Then I'd increase the mailbox to be just over the limit, and run the script again. If the mailbox is moved (and nothing else happens), the test passes, else the test fails.
And frankly, that's about all you need to test. Sysadmin scripts often do only one thing - or do one thing only if a condition is true. Which means there aren't many tests to do, and it's probably easier to just test it manually than to spend time figuring out how to automate it.