Added test for the proposed fix

This test is very simple, two nameserver instances are created:
    - ns4: master, with 'minimal-responses yes', authoritative
        for example. zone
    - ns5: slave, stub zone

The first thing verified is the transfer of zone data from master
to slave, which should be saved in ns5/example.db.

After that, a query is issued to ns5 asking for target.example.
TXT, a record present in the master database with the "test" string
as content.

If that query works, it means stub zone successfully request
nameserver addresses from master, ns4.example. A/AAAA

The presence of both A/AAAA records for ns4 is also verified in the
stub zone local file, ns5/example.db.

bin/tests/system/stub/ns4/example.db

@@ -0,0 +1,21 @@
+; 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 300	; 5 minutes
+@			IN SOA	ns4.example. hostmaster.example. (
+				2000042795 ; serial
+				20         ; refresh (20 seconds)
+				20         ; retry (20 seconds)
+				1814400    ; expire (3 weeks)
+				3600       ; minimum (1 hour)
+				)
+@        IN    NS      ns4
+ns4      IN    A       10.53.0.4
+         IN    AAAA    fd92:7065:b8e:ffff::4
+target   IN    TXT     "test"