Increase timeout in the rndc deadlock test

The timeout originally picked for "rndc status" invocations (2 seconds) in the test attempting to reproduce a deadlock caused by running multiple "rndc addzone", "rndc modzone", and "rndc delzone" commands concurrently causes intermittent failures of the "addzone" system test in GitLab CI. Increase the timeout to 10 seconds to make such failures less probable. Adjust code comments accordingly.

Closes #2763 (closed)