The headset jack has 3 terminals on it: the signal from the mic, the audio signal to the headphones and the common, which is the return/earth for both the mic and audio. The idea with the tester is simply to loop the mic signal back into the audio line, so that you test the mic, wires, PTT switch, jack and earphones all in one go. I'm assuming that it's essentially a case of wiring up the common terminal of a 1/2" jack receptacle to the the negative terminal of a 9V battery and wiring the mic and audio lines to the positive terminal, so the battery basically provides the power keep the mic signal going back to the earphones. Just not sure if I'd need something like a small amp or anything to boost the mic signal or not. If so, that'd make it a heap more complex!
Cheers,
John