Avoid running tests that require communication with the server if not needed #368
Description
Some of Nammu's tests send messages to the Oracc server and examine the response. This will fail if the server is not accessible, and running the tests very often may overload the server. We could designate some of them as "online-only", and only run them when we really want to check the connection with the server (Travis should always run them, presumably). If we don't want to connect to the server, or if we know it is down, we can just run the offline tests instead.