When double clicking spring.exe you can choose "Load Test.ssf".
Unfortunately AIs don't get a notification at all on loading so they'll just "hang". I've been told you can get around this by typing (in the ingame chat thing):
.cheat
.team 1
then select all AI's units and command them to Stop
Do the same for the other AIs (ie. '.team 2' and '.team 3'), then switch back to your team ('.team 0') and optionally disable cheating ('.cheat' again).