Resolve L2SS-1032 "Use constants in tests"

Closes L2SS-1032

tangostationcontrol/tangostationcontrol/clients/comms_client.py

@@ -15,6 +15,7 @@ class AbstractCommClient(ABC):
     def stop(self):
         """ Stop communication with the client. """
 
+    @abstractmethod
     def ping(self):
         """ Check whether the connection is still alive.