pfsync tests: check for the correct IP address

When checking if the state synced over we should look for
198.51.100.254, not 198.51.100.2. The test worked because the incorrect
address is a substring of the correct one, but we should fix it anyway.

Reported by: Naman Sood <naman@freebsdfoundation.org>
MFC after: 1 week