Mark espaloma test as xfail for issue #1940

[IAlibay]

Checklist

• All new code is appropriately documented (user-facing code must have complete docstrings).
• 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]

CPU tests are running here: https://github.com/OpenFreeEnergy/openfe/actions/runs/24516973152

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.34%. Comparing base (c60db06) to head (36e46a6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1941      +/-   ##
==========================================
- Coverage   94.62%   91.34%   -3.29%     
==========================================
  Files         206      206              
  Lines       18285    18286       +1     
==========================================
- Hits        17303    16703     -600     
- Misses        982     1583     +601     

Flag	Coverage Δ
fast-tests	91.34% <100.00%> (?)
slow-tests	?

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