Modularize tests

[FabianHofmann]

This PR introduces slight changes in code and major refactoring in the test scripts

[codecov]

Codecov Report

Base: 87.01% // Head: 87.61% // Increases project coverage by +0.59% 🎉

Coverage data is based on head (63cea5f) compared to base (f83907e).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
+ Coverage   87.01%   87.61%   +0.59%     
==========================================
  Files          13       13              
  Lines        2226     2220       -6     
  Branches      351      355       +4     
==========================================
+ Hits         1937     1945       +8     
+ Misses        210      196      -14     
  Partials       79       79              

Impacted Files	Coverage Δ
linopy/constraints.py	90.10% <100.00%> (+4.20%)	⬆️
linopy/expressions.py	87.19% <100.00%> (+0.03%)	⬆️
linopy/variables.py	91.08% <100.00%> (+0.14%)	⬆️

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