tests: sys.netpfil.pf.pfsync.defer fails in CI

sys.netpfil.pf.pfsync.defer has been failing for a while in CI now.
Disable it for now.

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

tests/sys/netpfil/pf/pfsync.sh

@@ -125,6 +125,10 @@ defer_body()
 {
 	pfsynct_init
 
+	if [ "$(atf_config_get ci false)" = "true" ]; then
+		atf_skip "Skip know failing test (likely related to https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=260460)"
+	fi
+
 	epair_sync=$(vnet_mkepair)
 	epair_in=$(vnet_mkepair)
 	epair_out=$(vnet_mkepair)