add Net::DNS prerequiste test

bin/tests/system/serve-stale/prereq.sh

@@ -0,0 +1,21 @@
+#!/bin/sh
+#
+# Copyright (C) 2011, 2012, 2014, 2016  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/.
+
+if $PERL -e 'use Net::DNS;' 2>/dev/null
+then
+    if $PERL -e 'use Net::DNS; die if ($Net::DNS::VERSION >= 0.69 && $Net::DNS::VERSION <= 0.74);' 2>/dev/null
+    then
+        :
+    else
+        echo "I:Net::DNS versions 0.69 to 0.74 have bugs that cause this test to fail: please update." >&2
+        exit 1
+    fi
+else
+    echo "I:This test requires the Net::DNS library." >&2
+    exit 1
+fi