test: explicit combo names and switch stable fast fixture

[gsprochette]

Description

This PR adds an id function to the test_full_integration_combo parametrization to make the test names explicit.
Before: flux_tiny_random-algorithms14-False-cmmd
After: flux_tiny_random-fora+stable_fast-False-cmmd

The stable diffusion fixture used for the stable_fast test has also been switched to a bigger one because the tiny-random model was too small for the available backends.

Related Issue

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 stable-fast test which now passes, and the stable-fast combo test passes and has an explicit name.

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

[begumcig]

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