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/setup.sh

@@ -15,3 +15,5 @@ SYSTEMTESTTOP=..
 copy_setports ns1/named.conf.in ns1/named.conf
 copy_setports ns2/named.conf.in ns2/named.conf
 copy_setports ns3/named.conf.in ns3/named.conf
+copy_setports ns4/named.conf.in ns4/named.conf
+copy_setports ns5/named.conf.in ns5/named.conf