Don't use the CUDA platform by default in tests

[IAlibay]

Fixes #1880

Checklist

• [N/A] All new code is appropriately documented (user-facing code must have complete docstrings).
• [N/A] Added a news entry, or the changes are not user-facing.
• Ran pre-commit: you can run pre-commit locally or comment on this PR with pre-commit.ci autofix.

Manual Tests: these are slow so don't need to be run every commit, only before merging and when relevant changes are made (generally at reviewer-discretion).

• GPU integration tests
• example notebook testing
• packaging tests: run this for any large feature PRs or PRs that add test data.

Developers certificate of origin

• I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[github-actions]

No API break detected ✅

[IAlibay]

Long CPU tests to verify CI passes here: https://github.com/OpenFreeEnergy/openfe/actions/runs/23305846325

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.18%. Comparing base (d69baa6) to head (c6d81c8).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1881      +/-   ##
==========================================
- Coverage   94.79%   91.18%   -3.62%     
==========================================
  Files         205      206       +1     
  Lines       17957    18249     +292     
==========================================
- Hits        17022    16640     -382     
- Misses        935     1609     +674     

Flag	Coverage Δ
fast-tests	91.18% <100.00%> (?)
slow-tests	?

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