TestPublisher should notice invalid re-use after termination

As noticed by @ idelpivnitskiy: in `RoundRobinLoadBalancerTest` the `serviceDiscoveryPublisher` is completed but later `onNext` is invoked. It violates the spec. `TestPublisher` should validate such misuse and the tests should create a new publisher.

Start by reading TestPublisher and RoundRobinLoadBalancerTest, especially the serviceDiscoveryPublisher lifecycle. Confirm how termination is represented and where a later onNext is currently accepted. Done means invalid reuse after termination is detected and the affected tests create a new publisher instead.

Written by the indexing model from the issue text.