pf tests: ensure syncookie does not create state

Test that with syncookies enabled pf does not create state for
connections before the remote peer has replied to the SYN|ACK message.

MFC after: 1 week
Sponsored by: Modirum MDPay
Differential Revision: https://reviews.freebsd.org/D31142

(cherry picked from commit 27ab791a55191c0b6503391d411303b042b41047)