Add prereq.sh scripts for system tests requiring perl mod DNS

prereq.sh mechanism for dependency detection is preferred since it works
for both legacy and pytest runner.

bin/tests/system/ixfr/prereq.sh

@@ -0,0 +1,22 @@
+#!/bin/sh
+
+# Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+#
+# SPDX-License-Identifier: MPL-2.0
+#
+# 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 https://mozilla.org/MPL/2.0/.
+#
+# See the COPYRIGHT file distributed with this work for additional
+# information regarding copyright ownership.
+
+. ../conf.sh
+
+if ! ${PERL} -MNet::DNS -e ''
+then
+	echo_i "perl Net::DNS module is required"
+	exit 1
+fi
+
+exit 0