pf tests: Only log critical errors from scapy

See a26e895f3d. Silence these new tests as
well.

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

tests/sys/netpfil/pf/fragcommon.py

@@ -27,6 +27,8 @@
 
 import argparse
 import os
+import logging
+logging.getLogger("scapy").setLevel(logging.CRITICAL)
 import scapy.all as sp
 import sys
 import time