system test covering NS randomization

system test covering NS randomization

Add randomizens system test which ensures that NS are randomly selected.
The test relies of the fact that `getaddresses_allowed()` logic won't
allow to query more than 3 NS at the top-level. The `example.` zone has
4 NS and the 3 formers are lame. As a result, if the resolved doesn't
randomize the NS selection, it will only quiery the 3 formers, which
won't give an answer, and fails. With randomization enabled, there is a
chance that the resolver queries the fourth NS, and gets the result.

(cherry picked from commit c67b52684f11652b07afaa75a917f6f0355dbca6)