pfctl parser tests

Copy the most important test cases from OpenBSD's corresponding
src/regress/sbin/pfctl, those that run pfctl on a test input file and check
correctness of its output. We have also added some new tests using the same
format.

The tests consist of a collection of input files (pf*.in) and
corresponding output files (pf*.ok). We run pfctl -nv on the input
files and check that the output matches the output files. If any
discrepancy is discovered during future development in the source
tree, we know that a regression bug has been introduced into the tree.

Submitted by: paggas
Sponsored by: Google, Inc (GSoC 2017)
Differential Revision: https://reviews.freebsd.org/D11322