pf tests: Fail the test if we can't set the rules

The test should fail if pf rules can't be set. This is helpful both
while writing tests and to verify that pfctl works as expected.

MFC after:	1 week
Event:		Aberdeen hackathon 2019

tests/sys/netpfil/pf/utils.subr

@@ -40,6 +40,10 @@ pft_set_rules()
 		printf "$1\n"
 		shift
 	done | jexec ${jname} pfctl -f -
+	if [ $? -ne 0 ];
+	then
+		atf_fail "Failed to set PF rules in ${jname}"
+	fi
 }
 
 pft_cleanup()