Adds mocking/testing example

[jcchavezs]

I used the library for a project and tried to come up with unit tests for it by mocking the name server tho I failed. It would be nice to have one example for that. I could come up with a PR for that if someone could help me to understand why my approach failed.

[miekg]

this might help: https://github.com/coredns/coredns/tree/master/plugin/pkg/dnstest