netpfil tests: improve pfsync_defer.py

Return different exit code depending on which failure was encountered.
The pfsync test expect a very particular failure, not just any.

MFC after: 1 week
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D38123

(cherry picked from commit 06012728beff45e94d58410eae7cda2ea980ef77)