pf tests: Add Scapy as a required program

The utils.subr file includes a couple of subroutines
(ping_dummy_check_request and ping_server_check_reply) that require
Scapy.

Add this requirement in the header of each test that makes use of them.

Reported by: Jenkins
Reviewed by: kp
Approved by: emaste (mentor)
Fixes: 07e070ef0869 ("pf: Add support for multiple source node types")
Differential Revision: https://reviews.freebsd.org/D49004