Cleaned up backend tests

[jesper-friis]

Description

Just some cleanup of the backend tests.

Added a pip freeze to CI tests for easier debugging.

Updated requirements in pyproject.toml as suggested by Francesca. Lower version range for dependencies has been tested manually.

Type of change

• [x] Bug fix and code cleanup
• [ ] New feature
• [ ] Documentation update
• [x] Testing

Checklist for the reviewer

This checklist should be used as a help for the reviewer.

• [ ] Is the change limited to one issue?
• [ ] Does this PR close the issue?
• [ ] Is the code easy to read and understand?
• [ ] Do all new feature have an accompanying new test?
• [ ] Has the documentation been updated as necessary?
• [ ] Is the code properly tested?

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #230 +/- ## ========================================= Coverage ? 78.41% ========================================= Files ? 18 Lines ? 1696 Branches ? 0 ========================================= Hits ? 1330 Misses ? 366 Partials ? 0 ```

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