Split long-running consumer integration test

PlaintextConsumerTest contains integration tests for the consumer. Since the introduction of the new consumer group protocol (KIP-848) and the new KafkaConsumer, this test has been parametrized to run with multiple combinations, making sure we test the logic for the old and new coordinator, as well as for the legacy and new KafkaConsumer.

This led to this being one of the longest-running integration tests, so in the aim of reducing the impact on the build times we could split it to allow for parallelization. The tests covers multiple areas of the consumer logic, in a single file, so splitting based on the high-level features being tested would be sensible and achieve the result wanted.