Test single DNS forwarder with knot's test tool
In reference to #123 @vmyslivec suggested to also add tests for every DNS forwarder. It would be button to run knot's tests (referenced in #123) for selected forwarder to suggest if user should or should not use it. This is less about suggestion but more about letting users to check if server (and specially their custom added servers) work correctly (if they should or should not choose them over just ISP's or ours ones).