tests: simplify diffusers fixture construction

[johannaSommer]

Description

This PR uses an pruna engine function we already have (determines the model class from the config json file) to simplify fixture construction for our tests with diffusers pipelines.

Related Issue

None.

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)
• This change requires a documentation update

How Has This Been Tested?

Ran the tests locally - they fail until a quick extension for the diffusers loading function is merged, hence a draft for now.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Additional Notes

None.

[nifleisch]

Nice cleanup, LGTM! 🙌