Support DLZ filesystem detection in feature-test

Do not use variable from configure to detect the feature.

bin/tests/system/dlz/prereq.sh

@@ -0,0 +1,19 @@
+#!/bin/sh
+#
+# Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+#
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+#
+# See the COPYRIGHT file distributed with this work for additional
+# information regarding copyright ownership.
+
+SYSTEMTESTTOP=..
+. $SYSTEMTESTTOP/conf.sh
+
+if ! $FEATURETEST --with-dlz-filesystem; then
+        echo_i "DLZ filesystem driver not supported"
+        exit 255
+fi
+exit 0