netpfil tests: Basic dummynet pipe test

Test dummynet pipes (i.e. bandwidth limitation) with ipfw. This is put
in the common tests because we hope to add dummynet support to pf in the
near future.

MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30380