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

tests/sys/netpfil/pf/ether.sh

@@ -642,6 +642,7 @@ short_pkt_head()
 {
 	atf_set descr 'Test overly short Ethernet packets'
 	atf_set require.user root
+	atf_set require.progs scapy
 }
 
 short_pkt_body()