Consider use of upstream kafka-client mocks in testkit and project unit tests

[seglo]

Short description

Apache Kafka provides a MockConsumer and MockProducer implementation. We should investigate if we can reuse and wrap these mocks in our own client mocking solution.

Details

We mock the Kafka clients in several tests (ProducerSpec and ConsumerSpec). Investigate if the upstream mocks could be used in place and if it provides any value.

https://kafka.apache.org/23/javadoc/org/apache/kafka/clients/consumer/MockConsumer.html https://kafka.apache.org/23/javadoc/org/apache/kafka/clients/producer/MockProducer.html