pf tests: Test the match keyword

The new match keyword can currently only assign queues, so we can only
test it with ALTQ.
Set up a basic scenario where we use 'match' to assign ICMP traffic to a
slow queue, and confirm that it's really getting slowed down.

MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31116