Add integration tests for benchmarks

For all our support stream processing engines, we would like to provide integration tests, which verify that some data is processed by the benchmark. These tests should not focus on the correctness of computations, for this purpose unit tests (e.g. using Kafka Streams' or Flink's testing frameworks) are better suited.

• Testcontainers can be used to set up Kafka and Theodolite load generator
• Although focused on Kafka Streams, this blog post explains usage of Testcontainers for Kafka: https://www.confluent.io/blog/testing-kafka-streams/
• Kafka JUnit might be used to simplify interaction with Kafka.
• For UC1, we have to verify that the Logger produces some messages.
  • https://dzone.com/articles/unit-testing-log-messages-made-easy
  • https://www.baeldung.com/junit-asserting-logs
• For UC2-4 we will check Kafka's "output" topic.