The least intrusive method for testing would probably be to create a test virtual server. If you don't have free IP addresses, you could use an existing virtual server IP address with a new port.
Also, you can test some rules by putting the commands in the RULE_INIT event and not associate the rule with any VIP. This event is triggered the rule is initialized (like when the rule is created or updated. This trick wouldn't work if you're trying to evaluate actual request info or test commands that are specific to another event. But it does give you a way to test things like string manipulations.
Lastly, you can test non-F5 specific TCL commands with a TCL shell. ActiveState has a free version:
http://www.activestate.com/Products/ActiveTcl/
Or you can use cygwin's tclsh.exe:
http://www.cygwin.com/mirrors.html
Per SOL6091, F5 based their iRules language on TCL version 8.4.6.
Aaron