pf tests: fix plfow:v6 after scapy-2.6.0 update

Avoid receiving ICMPv6 destination unreachable packets, because scapy will
attempt to parse the pflow information from those (partial) packets as well, and
then get confused.

MFC after:	1 week
Sponsored by:	Rubicon Communications, LLC ("Netgate")

tests/sys/netpfil/pf/pft_read_ipfix.py

@@ -60,7 +60,7 @@ def parse_ipfix(p):
 		c = datafl.payload
 
 def receive(recvif, recvport):
-	pkts = sp.sniff(iface=recvif, timeout=65)
+	pkts = sp.sniff(iface=recvif, timeout=65, filter="udp port 2055")
 
 	if len(pkts) == 0:
 		print("No data")