> why is there no in-memory kafka server that I can use for simple testing purposes.

Take a look at Debezium's KafkaCluster, which is exactly that: https://github.com/debezium/debezium/blob/main/debezium-core....

It's used within Debezium's test suite. Check out the test for this class itself to see how it's being used: https://github.com/debezium/debezium/blob/main/debezium-core...