pf tests: test pflow NAT state information

pflow(4) now also exports NAT session creation/destruction information.
Test that this works as expected.

While here improve the parsing of ipfix (i.e. pflowproto 10) a bit, and
check more information for the existing state information exports.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D43117