Automatically test hostname leaks

Originally created by @intrigeri on #7712 (Redmine)

It would be good to at least detect hostname leaks caused by the DHCP client, to catch regressions such as #7688 (closed) earlier. Presumably, our existing test suite infrastructure that can capture all network traffic and inspect it could be reused here.

(We now have a step to check this as part of our manual test suite, but adding more and more pain there is no real solution.)

Feature Branch: test/7712-hostname-leaks

Related issues

• Related to #7688 (closed)