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