Regression test for the diagnostics dump feature

[mflendrich]

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Problem Statement

#1308 introduced config dumps that later regressed in KIC 3.0 (#5270).

Let's prevent this regression from happening ever again.

Proposed Solution

Implement a test for the diagnostics server verifying that the server is indeed started on the right port and that it provides meaningful data.

The test should be able to catch the regression fixed by #5270.

Additional information

No response

Acceptance Criteria

• a test validating the presence and core functionality of the diagnostics dump exists and runs on CI
• the test retroactively covers the regression fixed by fix: restore standard diagnostics port #5270