Fix flakiness in the doth reconfig test

Due to the fact that the primary nameserver creates a lot of TLS contexts, its reconfiguration could take too much time on the CI, leading to spurious test failures, while in reality it works just fine (a recent example)

This commit adds a separate instance for this test which does not use ephemeral keys (these are costly to generate) and creates minimal amount of TLS contexts.

Also, see #2983 (closed) and the corresponding MR. This problem is a different manifestation of the mentioned issue.