netpfil tests: extend dummynet tests to pf
Now that pf can also use dummynet we should extend the existing dummynet
tests to also test it when used with pf.
Reviewed by: donner
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31905