pf tests: require scapy for ether:short_pkt

The pft_ether.py script requires both python and scapy to be installed.
Check for this so we properly skip the test when it is unavailable.

Reviewed by: kp
Fixes: 07ffa50ba075d ("pf tests: test short packets")
Differential Revision: https://reviews.freebsd.org/D36561