Add basic NAT test for pf, ipf and ipfw

Add common firewall NAT tests for pf, ipf and ipfw (using both in-kernel and
userspace NAT).

Submitted by: Ahsan Barkati
Sponsored by: Google, Inc. (GSoC 2019)
Reviewed by: kp
Approved by: bz (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D21199