add testing to m3gnet potential training example

[lbluque]

Checklist

• Google format doc strings added. Check with ruff.
• Type annotations included. Check with mypy.
• Tests added for new features/fixes.
• If applicable, new classes/functions/modules have duecredit @due.dcite decorators to reference relevant papers by DOI (example)

Tip: Install pre-commit hooks to auto-check types and linting before every commit:

pip install -U pre-commit
pre-commit install

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (543c3e0) 98.00% compared to head (fe4aa05) 98.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #179   +/-   ##
=======================================
  Coverage   98.00%   98.00%           
=======================================
  Files          28       28           
  Lines        1901     1901           
=======================================
  Hits         1863     1863           
  Misses         38       38           

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[shyuep]

Thanks.