netpfil tests: Add forward test for the three firewalls
Submitted by: Ahsan Barkati
Reviewed by: kp
Sponsored by: Google, Inc. (GSoC 2019)
Differential Revision: https://reviews.freebsd.org/D21321
(cherry picked from commit 39cae0d5ed570804b4e689946f131447597122b8)