netpfil tests: skip checksum verification in pft_ping.py

From scapy 2.6.0 it now removes (sets to None) the IP checksum in its defrag
code. We typically ask for IPv4 or IPv6 defragmentation, which causes check
function failures.

Just skip the checksum verificiation here, because we're going to notice
incorrect checksums even if the tests don't directly verify it.

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