Consider adding fast tests without real vmnet or real vm (qemu, lima)

[nirs]

If we mock the few vmnet functions we use, we can run socket_vment or unit tests sending fake packet from "vmnet" or from the "vm".

fake vm <-socket-> socket_vment -> fake_vmnet

The fake vm can open the socket and echo packets received from the socket back to the socket.
The fake vmnet can forward packets from socket_vmnet to the test code, and allow the test code to send packets.

We can simulate one or more vms connected to socket_vmnet.