dnsdist: Add API regression tests

dnsdist: Add API regression tests

So we'll notice if we break the web server or the API.
This commit also cleans up the responses counter usage.
Because the counters are written to by the responder threads,
they are updated at the class level. We do not want to alter
them at the instance level, because that would create a new variable
at this level, disconnected from the other one, but we can still
read them from the instance level using 'self'.