pf tests: test fast port re-use with syncookies

When a src/dst ip/port tuple is re-used before the pf state fully
expires we clean up the state and create a new one, unless syncookies
are enabled.

Test this, by running two back-to-back nc sessions, with a fixed source
port. Move the interface and IP to a different (vnet) jail, to trick the
network stack into letting us do this.

MFC after: 2 weeks
Event: Aberdeen hackathon 2022
Differential Revision: https://reviews.freebsd.org/D36886