jail: skip sysctl_security_jail_children if jail binary missing

Approved by:	lwhsu
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D44010

sys/kern/sysctl_security_jail_children.sh

@@ -33,6 +33,9 @@ max_cur_head()
 {
 	atf_set descr 'Test maximum and current number of child jails'
 	atf_set require.user root
+	if ! which -s jail; then
+		atf_skip "This test requires jail"
+	fi
 }
 max_cur_body()
 {