pf tests: test ALTQ CBQ on top of if_vlan

The main purpose of this test is to verify that we can use ALTQ on top
of if_vlan, but while we're here we also exercise the CBQ code. There's
already a basis test for HFSC, so it makes sense to test another
algorithm while we test if_vlan.

Reviewed by: donner
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31649