pf tests: verify checksum offload handling

Disable epair's checksum offload on one interface in a nat test, to ensure
that pf does the right thing in scenarios where one interface offloads checksum
handling and one does not.

We trust the TCP (or ICMP) checksum verification code to drop the packet if pf
does it wrong, which will cause the test case to fail.

Suggested by: tuexen
Sponsored by: Rubicon Communications, LLC ("Netgate")