Adding folder structure to tests

[ciguaran]

Solves issue #480 . I have moved entire folder files without splitting any of them into folder pieces, matching them to the modules in the source code.

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[codecov]

Codecov Report

Merging #483 (94db89c) into main (f16fd05) will decrease coverage by 0.01%.
The diff coverage is n/a.

❗ Current head 94db89c differs from pull request most recent head e50e699. Consider uploading reports for the commit e50e699 to get more accurate results

@@            Coverage Diff             @@
##             main     #483      +/-   ##
==========================================
- Coverage   99.16%   99.16%   -0.01%     
==========================================
  Files          48       48              
  Lines        1915     1914       -1     
==========================================
- Hits         1899     1898       -1     
  Misses         16       16              

Impacted Files	Coverage Δ
blackjax/util.py	100.00% <0.00%> (ø)
blackjax/optimizers/lbfgs.py	100.00% <0.00%> (ø)

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[rlouf]

Looks good to me!