Fix flakiness in the doth reconfig test

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.

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.