Make test helpers available

[jeffh]

Thanks for clj-kafka, it's been great to use. :+1:

When it comes to testing our microservices, we've been copying clj-kafka's test helpers to our projects. It seems generally useful for writing tests that integrate through kafka.

Would it be appropriate to make this available in clj-kafka or a separate test helper library?

[retnuh]

Same with us, I've copied that into our project and tweaked a little bit...