pf tests: Test multi-wan rdr

This replicates an issue observed on pfSense: https://redmine.pfsense.org/issues/11436

In essence, reply-to is needed to ensure that connections always leave
the WAN interface they came in on, but this confused the state tracking.

MFC after: 2 week
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit f37667e2359245ad123fd775c072fd82c81bc476)