If VIMAGE is present we can start jails with their own pf instance. This
makes it fairly easy to run tests.
For example, this basic test verifies that drop/pass and icmp
classification works. It's a basic sanity test for pf, and hopefully an
example on how to write more pf tests.
The tests are skipped if VIMAGE is not enabled.
This work is inspired by the GSoC work of Panagiotes Mousikides.
Differential Revision: https://reviews.freebsd.org/D12580
