WIP: conncheck: add a self test function

I'm not sure what could be the best way to integrate this piece of code, but I think it would be interesting to have some kind of "self test" or "sanity check" function like this one, that would periodically verify some invariant properties while the connection check is running. This helped me a lot to catch bugs, as soon as they show up, in addition to commit bellet/libnice@42800b76