There is no practical way to test the power supply using software such as the AHT. Testing requires, at a minimum, a Volt/Ohm/Ammeter and a system specification. Unless you are a moderately skilled technician with the proper test equipment replacing the power supply would probably be cheaper than the labor to test it.
Any reputable Apple Certified Repair center can provide you with the specs, the replacement power supply, and install it as well. Power supplies are not considered user replaceable parts.
