tests: flux variant probe testing

[psychedelicious]

Summary

• Add tests for the util added in #7862.
• Add stripped models for FLUX variants.

There's a problem with CI - perhaps related to the GH CI runner for macOS being x86-64 instead of Apple Silicon. Haven't investigated deeply yet, but I cannot reproduce the issue locally on my M1 Pro MacBook.

The commits in this PR were split off from #7862 so we could get the user-facing bugfix released.

Related Issues / Discussions

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QA Instructions

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Merge Plan

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Checklist

• [x] The PR has a short but descriptive title, suitable for a changelog
• [x] Tests added / updated (if applicable)
• [ ] Documentation added / updated (if applicable)
• [ ] Updated What's New copy (if doing a release after this PR)