Common testing: move unit test utils to testing_utils

[sfc-gh-turbaszek]

Pre-review checklist

• [ ] I've confirmed that instructions included in README.md are still correct after my changes in the codebase.
• [ ] I've added or updated automated unit tests to verify correctness of my new code.
• [ ] I've added or updated integration tests to verify correctness of my new code.
• [ ] I've confirmed that my changes are working by executing CLI's commands manually.
• [ ] I've confirmed that my changes are up-to-date with the target branch.
• [ ] I've described my changes in the release notes.
• [ ] I've described my changes in the section below.

Changes description

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