Use spawn start method for multiprocessing inside the test to avoid deadlock/timeout

[yuxuanzhuang]

Fixes #5191

Changes made in this Pull Request:

• use a spawn start method inside the test to avoid deadlock

LLM / AI generated code disclosure

LLMs or other AI-powered tools (beyond simple IDE use cases) were used in this contribution: no

PR Checklist

• Issue raised/referenced?
• Tests updated/added?
• Documentation updated/added?
• package/CHANGELOG file updated?
• Is your name in package/AUTHORS? (If it is not, add it!)
• LLM/AI disclosure was updated.

Developers Certificate of Origin

I certify that I can submit this code contribution as described in the Developer Certificate of Origin, under the MDAnalysis LICENSE.

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📚 Documentation preview 📚: https://mdanalysis--5213.org.readthedocs.build/en/5213/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.72%. Comparing base (4c12f15) to head (5d30d48).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #5213   +/-   ##
========================================
  Coverage    92.72%   92.72%           
========================================
  Files          180      180           
  Lines        22475    22475           
  Branches      3190     3190           
========================================
  Hits         20841    20841           
  Misses        1177     1177           
  Partials       457      457           

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[yuxuanzhuang]

Tested twice and passed.