Add a somewhat more extensive pfsync defer mode test. Ensure that pfsync
actually delays the state creating packet until after it has sent the
pfsync update and given the peer time to create the state.

Ideally the test should validate the pfsync state update and generate an
ack message, but to keep the test simple we rely on the timeout of the
deferred packet instead.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")