pf tests: make pfsync:defer test more robust

Set the defer timeout to 2.5 seconds, and only make sure that there's at
least a second of delay between the pfsync packet and the ping packet.
The intent is to allow the test to pass even when there's considerable
jitter on the timing measurement (e.g. when the test runs in a VM, such
as during the CI tests).

Allow the test to run in CI.

Sponsored by: Rubicon Communications, LLC ("Netgate")