Allow Manual Running of Slow Tests

[mikemhenry]

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/.

[mikemhenry]

Once we merge this in I will add a page to our wiki https://github.com/OpenFreeEnergy/openfe/wiki showing how do run the slow tests on a PR, I can't make the screenshots until after it gets merged in.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d011065) 90.97% compared to head (a16ae16) 90.97%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #649   +/-   ##
=======================================
  Coverage   90.97%   90.97%           
=======================================
  Files         127      127           
  Lines        8679     8679           
=======================================
  Hits         7896     7896           
  Misses        783      783           

Flag	Coverage Δ
fast-tests	90.97% <ø> (ø)

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