RFE: add checks for encrypted DNS setup

[antoniotorresm]

We could add some tests to ensure that the encrypted DNS setup is working correctly, for example:

• forwarder is set to unbound address
• unbound service should be enabled
• verify unbound config, both in server and client (client config should point to master)
• client should use local unbound as DNS server, and this should point to master DNS

These are just some ideas, we should explore further.