Modern DC to DC PSU don't need a load on 5V "rail". All the AC power is converted into 12V and other voltages (like 5V) is supplied from the 12V.
Some older PSU:
|----- 12VDC
120V/240VAC ------------ 5VDC
|----- 3.3VDC
Modern PSU:
120V/240VAC ------- 12VDC ----------- 12VDC
|------------- 5VDC
|------------- 3.3VDC
I very very very seriously doubt your voltage tester is pulling 100W. Is it as hot as touching a 100W light bulb? The voltage it is using depends on where you have it connected. If you have it connected to a MOLEX (4 pin) connector then it is putting a load on +5V & +12V rails. If I had to guess I would say it is probably <5W.