bridge tests: Test that we also forward on some interfaces

Ensure that we not only block on some interfaces, but also forward on
some. Without the previous commit we wound up discarding on all ports,
rather than only on the ports needed to break the loop.

MFC after: 1 week
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D28917

(cherry picked from commit 7a4dbffa4205fc274b4884a6332d4831c5791320)