You mean test the low Z input?
I have 1/4 to XLR adaptors in my kit. I have some wired pin 3 hot (2 ground), and some wired pin 2 hot (3 ground). Pick one, and plug it into the XLR on the unit. It will apply a signal to either pin 2 or 3, and ground the other hot pin. That will work well enough to sort out a problem. Ideally check both ways.
Note, I am still using pins 2 and 3 rather than chassis ground. I am grounding one of those two pins.
But really... I have my trusty SM58 on a three foot XLR cord sitting next to my bench. If you only want to test functionality of the circuit, most any low Z XLR mic will work. Just bite the bullet and get some sort of mic for your bench.