Better test timeouts in UnreachableNodeJoinsAgainSpec, see #3285

@drewhk and I analyzed the log and we could see that the Welcome message was not delivered, but sent to deadLetters after 5 seconds, when the system was shutdown. We don't have an explanation why the Welcome message was not sent within 5 seconds. We have run the test again repeatedly with this new timeout and have not seen any failures. I will keep the job running, but if we don't see any more failures we must close it as not reproducible.

This is a good change anyway.