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