Perform basic resolution checks with a mirror zone in use

Make ns3 mirror the "root" zone from ns1 and query the former for a
properly signed record below the root.  Ensure ns1 is not queried during
resolution and that the AD bit is set in the response.

bin/tests/system/mirror/ns2/example.db.in

@@ -0,0 +1,14 @@
+; 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.
+
+$TTL 3600
+@		SOA	ns2 hostmaster 1 3600 1200 604800 3600
+@		NS	ns2
+ns2		A	10.53.0.2
+foo		A	127.0.0.1