So I'm going to ask again.

How can I set up a loopback test for #MIDI over #usb

ie no #DIN5 cable. For testing purposes I want a way to take midi out on a usb cable and plug it in to midi in on a usb cable without latency in the loopback connection (bcause I'm tetsing the I/O #jitter of the host computer) and I don't see how it can be done.

I don't trust midi over usb. DIN5 midi is kinda crap, but its known crap with well understood limitations.