Not likely. The problem is that the PSU will shut down (self protection) if it has less that its minimum current draw. It has to do this because the voltage regulation loop goes unstable if it was designed to have a higher minimum current draw. If the PSU didn't shut down, it would fry things, including itself. Really bad.
To test the PSU works at 0.5A would end up shutting down the computer if the PSU didn't work at 0.05A... exactly what happens today. Unfortunately, at that point it is too late to tell the user "your PSU is old" or to quickly connect a resistor across the +12v rails to work around the problem.
As an experiment, find an old PSU and plug it into the wall (nothing connected to the power output connectors). It won't power on. If you connect a 2 ohm 10W resistor (or two 1 ohm 5W resistors in series) into the "hard drive" Molex connector +12v (yellow) to ground (black), it should start up and be happy.
Disclaimers:
1) The resistor will become quite warm quite quickly (6W), watch your fingers.
2) If you damage something, it is a learning experience, not my fault.
The most likely outcome though is that the supply will just power on and idle without any load. Rapid transients on the rails (say from 100W to ~0W) might cause the over voltage crowbar to trip, though.
There actually shouldn't be any issue at all with the motherboard testing this. Just set a flag in the EEPROM that your testing the C6/C7 states, and try it, if the CPU exits C6/C7 with the flag set, it passed. If the system goes through a normal boot with the C6/C7 flag still set, it knows that it failed to exit C6/C7 (the supply cut out). This is basically the same procedure many motherboards use to detect if the over-clocking settings you tried were too agressive and caused the system to fail to boot.
It seems to me that in any compliant PSU, not drawing enough current from the +12V2 rail shouldn't trigger a shutdown that results in a loss of power on the +5Vsb rail, so motherboard makers just have to ensure that whatever testing circuitry they use can run off just the standby power.
Ah, yes, the other side of the Halting Problem: not only can you not tell if your program will ever halt, but you also can't tell if your program has been externally halted.
Computers were doing that since forever though. Remember all those "your computer wasn't shutdown correctly, do try to do better next time would you" messages? Just write a bit into CMOS before doing a test, then assume the test failed if the bit is still set on next power up.
