Reduce testing time for gmso implementation

[justinGilmer]

PR Summary:

At the moment, loading in large forcefield XML files so that GMSO can be used as the backend data structure has some performance issues.

This introduces a fixture called oplsaa which loads the oplsaa.xml forcefield once and can be used wherever needed in the test_gmso_forcefield.py. This currently reduces the entire testing time from 573 seconds to 348 seconds. I used: time python -m pytest -v foyer/tests

PR Checklist

---

• [x] Includes appropriate unit test(s)
• [x] Appropriate docstring(s) are added/updated
• [x] Code is (approximately) PEP8 compliant
• [ ] Issue(s) raised/addressed?

[codecov[bot]]

Codecov Report

Merging #437 (5fc0f06) into master (fc44f80) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 5fc0f06 differs from pull request most recent head fadf4f0. Consider uploading reports for the commit fadf4f0 to get more accurate results

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  Coverage   73.58%   73.58%           
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  Files          17       17           
  Lines        1802     1802           
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  Hits         1326     1326           
  Misses        476      476