Refactor ShareConsumerTest

1. `consumeMessages` should return `messagesConsumed` instead of wrapping it in `CompletableFuture`
2. replace `fail` by `assertDoesNotThrow` or just throw exception
3. `produceMessages`should return `messagesConsumed` instead of wrapping it in `CompletableFuture`
4. use `CompletableFuture.runAsync()` instead of ExecutorService. We don't use the interrupt so it is fine to use default executor