chore(refactor, test): Move test utils into own subdirectory

[aaron-hopkinson]

Description

Grouped together all dataset creation testing related utilities (which aren't themselves tests) into a utils subdirectory.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Issue Number

Code Compatibility

• I have performed a self-review of my code

Code Performance and Testing

• I have added tests that prove my fix is effective or that my feature works
• I ran the complete Pytest test suite locally, and they pass

Dependencies

• I have ensured that the code is still pip-installable after the changes and runs
• I have tested that new dependencies themselves are pip-installable.

Documentation

• My code follows the style guidelines of this project
• I have updated the documentation and docstrings to reflect the changes
• I have added comments to my code, particularly in hard-to-understand areas

Additional Notes

[b8raoult]

Can you take the opportunity of that PR to split tests in units/integration.

[aaron-hopkinson]

Can you take the opportunity of that PR to split tests in units/integration.

@b8raoult following the conversations the other week – I suspect the definition of unit vs integration vs other tests might generate some discussion, so I've made this change as a separate PR so we can tackle this independently.

[HCookie]

Resolve the conflicts then we will merge