netpfil tests: Add functions for testing routing scenarios

Many pf tests use identical setup where one jail is a router and
optionally another jail is a server. Add functions to create such jails
for IPv6 and IPv4 and functions to perform tests on such setup.

Add tests using those functions: scrub actions, routing table, tcp
sequence number modulation.

Reviewed by: kp
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D38126