netpfil tests: Add the set ToS test for ipfw and pf

netpfil tests: Add the set ToS test for ipfw and pf

This test tests the following:

 - The firewall is able to set the tos bits
 - The firewall is able to set the DSCP bits when EN bits is already set and
   the EN bits remains unchanged.
 - The firewall is able to drop the packets based on ToS value

Submitted by:	Ahsan Barkati
Reviewed by:	kp
Sponsored by:	Google, Inc. (GSoC 2019)
Differential Revision:	https://reviews.freebsd.org/D21305

(cherry picked from commit df3d236d96456f1c8400c7898045eae3ea93232d)