test: ☑️ add Python unit test

[rodneylab]

Description

Add Python unit test.

Type of change

• Testing

How Has This Been Tested?

• cargo test run with all tests passing
• python -m unittest -v tests/test_subsetter_tool.py

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.75000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 30.68%. Comparing base (162e506) to head (439818b).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
subsetter_tool.py	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #18      +/-   ##
==========================================
+ Coverage   29.11%   30.68%   +1.57%     
==========================================
  Files           5        5              
  Lines        1216     1245      +29     
==========================================
+ Hits          354      382      +28     
- Misses        862      863       +1     

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