pf tests: test NAT-ed ICMP errors

Ensure that the ICMP error is returned with the correct
source and destination addresses.

MFC after: 3 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D32572

(cherry picked from commit 30276ef12cbb47cdd302741096262041be30ec11)