pf tests: Remove Scapy as a required program

Scapy is not needed to run this test. Remove it from the required
programs list.

Reviewed by: kp
Approved by: emaste (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D49006

(cherry picked from commit 1f1963bd32d669f89ea3b044636de0cb7a33134b)