pf tests: Improve tests for af-to

NAT64 performed on inbound direction bypasses outbound filtering
and creates only a single state which is bound to the outbound
interface. Extend TCP NAT64 tests to cover all combinations
of inbound/outbound rule and floating/if-bound states.

Reviewed by: kp
Approved by: kp
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D51788