Add unit tests for ClusterConnectionStates

There are no unit tests for the ClusterConnectionStates object.

I've created tests to:

• Cycle through connection states and check correct properties during the process
• Check authentication exception is correctly stored
• Check that max reconnect backoff is not exceeded during reconnects
• Check that removed connections are correctly reset

There is currently no test that checks whether the reconnect times are correctly increased, as that is still being fixed in KAFKA-6101 .