pf tests: test SCTP 'return'

Ensure that we send a correct abort message for 'block return' rules.

Test this by validating that nc doesn't sit around waiting for a
connection. It should give up immediately when it receives the abort.

MFC after: 3 weeks
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D40865

(cherry picked from commit 2d42aa9d7ba64fe7166f76234a595637f0c212cf)