test: Fixed TensorFlow predictor unittest

[fg-mindee]

This PR reduces the number of crops processed in unittests by using a pretrained detection model. It changed the number of crops from several thousands to a maximum of a few dozens.

It also prevents degraded localization candidates, and will hopefully fix the ongoing issue with current PR unittests in TensorFlow.

Any feedback is welcome!

[codecov]

Codecov Report

Merging #767 (2cf3965) into main (f2f1f21) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #767      +/-   ##
==========================================
- Coverage   96.19%   96.15%   -0.05%     
==========================================
  Files         129      129              
  Lines        4810     4810              
==========================================
- Hits         4627     4625       -2     
- Misses        183      185       +2     

Flag	Coverage Δ
unittests	96.15% <ø> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
doctr/models/recognition/predictor/tensorflow.py	91.17% <0.00%> (-5.89%)	⬇️

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[charlesmindee]

Thanks for the PR!