tests: add test for ipfw fwd localaddr,port

[glebius]

Summary

Checks basic forward to local address, also with presence of
a listener that matches original port (see aab8c844b91). Based
on non-ATF test case written by Pavel Polyakov.

[kp]

Looks sane to me.

I believe this is the first ipfw-specific test. We have a few ipfw tests already in netpfil/common, but those tests also test pf (and sometimes ipf). 'fwd' is a little less generic, so it makes sense to separate that.