pf tests: basic SCTP multihoming test

The SCTP server will announce multiple addresses. Block one of them with
pf, connect to the other have the client use the blocked address. pf
is expected to have created state for all of the addresses announced by
the server.

In a separate test case add the secondary (client) IP after the
connection has been established. The intent is to verify the
functionality of the ASCONF chunk parsing.

MFC after: 3 weeks
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D41638