System tests send out DNS queries to non-local IP addresses

Some named instances used in system tests send outbound queries to non-local IP addresses. This should not happen as test traffic should be confined to the host running the tests.

This issue was severely aggravated by !145 (merged), which changed the default value for the dnssec-validation setting to auto. Since this change also applies to servers configured with recursion no;, a lot more named instances used during system tests started sending ./NS and ./DNSKEY queries over non-standard ports towards real root servers. (See also #949.)