pf tests: test 'rdr' for SCTP

Explicitly test that we cannot change the port number with rdr.
That's not a desireable feature on SCTP, because it could break
multihomed connections.

MFC after: 3 weeks
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D40868

(cherry picked from commit 4369a57514f227c989d4de87d1ce54470279ba41)