pf tests: ALTQ priority test

Test that ALTQ can prioritise one type of traffic over another. Do this
by establishing a slow link and saturating it with ICMP echos.
When prioritised TCP connections reliably go through. When not
prioritised TCP connections reliably fail.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit cd46399b9ccf04f6ec00a532e52c8b1edb007af7)