pf tests: Forwarding syncookie test

Test syncookies on a forwarding host. That is, in a setup where the
machine (or vnet) running pf is not the same as the machine (or vnet)
running the server it's protecting.

MFC after: 1 week
Sponsored by: Modirum MDPay
Differential Revision: https://reviews.freebsd.org/D31141