Add more test cases:
Add more test cases:

* overriding stripping of atom stereochemsitry
* cases in which it should not be stripped, i.e. not pyrimidal nitrogens
  * 4-coordinated nitrogen
  * planar (3-coordinated) nitrogen

open-forcefield-group/openforcefield,openforcefield/openff-toolkit,open-forcefield-group/openforcefield,open-forcefield-group/openforcefield,openforcefield/openff-toolkit