test: Added unittest for crop extraction

[fg-mindee]

This PR simply adds a unittest for crop extraction

[codecov]

Codecov Report

Merging #104 (049aaf7) into main (a2d5c13) will increase coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
+ Coverage   97.76%   97.88%   +0.11%     
==========================================
  Files          27       28       +1     
  Lines         895      897       +2     
==========================================
+ Hits          875      878       +3     
+ Misses         20       19       -1     

Flag	Coverage Δ
unittests	97.88% <ø> (+0.11%)	⬆️

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Impacted Files	Coverage Δ
doctr/__init__.py	100.00% <0.00%> (ø)
doctr/models/_utils.py	100.00% <0.00%> (+7.14%)	⬆️

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