remove slot name redundancy in get_slots_by_enum method

[sierra-moxon]

to avoid doc displays like this:
https://biolink.github.io/biolink-model/ChemicalOrGeneOrGeneProductFormOrVariantEnum/
(Slots section)

While I was in there, I also converted the tests associated with this method to pytest and added the schema as a fixture.

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (a39fd65) 62.12% compared to head (801973d) 61.99%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
linkml_runtime/utils/schemaview.py	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #294      +/-   ##
==========================================
- Coverage   62.12%   61.99%   -0.13%     
==========================================
  Files          63       63              
  Lines        8477     8480       +3     
  Branches     2175     2176       +1     
==========================================
- Hits         5266     5257       -9     
- Misses       2601     2612      +11     
- Partials      610      611       +1     

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[sujaypatil96]

LGTM 🚀 removes redundant slots from being returned by get_slots_by_enum() method.