Add common firewall test suite

Add a common test suite for the firewalls included in the base system. The test
suite allows common test infrastructure to test pf, ipfw and ipf firewalls from
test files containing the setup for all three firewalls.

Add the pass block test for pf, ipfw and ipf. The pass block test checks the
allow/deny functionality of the firewalls tested.

Submitted by: Ahsan Barkati
Sponsored by: Google, Inc. (GSoC 2019)
Reviewed by: kp
Approved by: bz (co-mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D21065

(cherry picked from commit f97a8a36153a9773d9f93018d66a7de2d050a59a)