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

tests/sys/netpfil/pf/pfsync.sh

@@ -172,7 +172,7 @@ defer_body()
 		"set skip on ${epair_sync}a" \
 		"pass keep state"
 
-	atf_check -s exit:1 env PYTHONPATH=${common_dir} \
+	atf_check -s exit:3 env PYTHONPATH=${common_dir} \
 		$(atf_get_srcdir)/pfsync_defer.py \
 		--syncdev ${epair_sync}b \
 		--indev ${epair_in}b \

tests/sys/netpfil/pf/pfsync_defer.py

@@ -117,15 +117,15 @@ def main():
         sys.exit(1)
 
     if not got_ping:
-        sys.exit(1)
+        sys.exit(2)
 
     if got_pfsync > got_ping:
-        sys.exit(1)
+        sys.exit(3)
 
     # Deferred packets are delayed up to 20ms (unless the pfsync peer, which we
     # don't have here, acks their state update earlier)
     if (sent_ping + 0.020) > got_ping:
-        sys.exit(1)
+        sys.exit(4)
 
 if __name__ == '__main__':
     main()