ipsec tests: Skip if ipsec.ko is not loaded

As of r347410 IPSec is no longer built into GENERIC. The ipsec.ko module must
be loaded before we can execute the IPSec tests.

Check this, and skip the tests if IPSec is not available.

tests/sys/netipsec/tunnel/utils.subr

@@ -8,6 +8,10 @@
 
 ist_init()
 {
+	if ! sysctl -q kern.features.ipsec >/dev/null ; then
+		atf_skip "This test requires ipsec"
+	fi
+
 	vnet_init
 }