Improve integration tests

[mmacai]

Description

• Re-introduced default randomized value for Kafka group ID
• Removed fixed group id from integration tests -> now it's using randomized group ID
  • Reason: prevent consumption of old messages in new test runs due to failing tests

What to look out for

• run integration tests several times

Process

The goal is to improve not only the code in this PR but also our skills! The "rules":

• The review is considered "done" as soon as all reviewers have added their review, and all their comments have been addressed.
• For knowledge-sharing reviews, each reviewer should "approve" the PR after studying its content.
• After the approval, the merge is concluded by the developer.

Have fun!