IIHS uses cars that aren't running and don't have fluids in them. They don't want to clean up that mess (or have the car destroyed by fire). They do test fuel system integrity though. I'm not sure if they use some other fluid (water + dye) or something like pressurized air or leakdown test after the fact.

I guess you could engage in other antics but none of them would be all that effective since the car is towed by a cable until it's a few feet from whatever it's hitting.