MFC r341999:

pf tests: NAT exhaustion test

It's been reported that pf doesn't handle running out of available ports
for NAT correctly. It freezes until a state expires and it can find a
free port.
Test for this, by setting up a situation where only two ports are
available for NAT and then attempting to create three connections.

If successful the third connection will fail immediately. In an
incorrect case the connection attempt will freeze, also freezing all
interaction with pf through pfctl and trigger timeout.

PR: 233867