More correctness tests for symmetric coloring

[gdalle]

• Add star coloring tests for some matrices in the SuiteSparse collection, following Tables 4.1 and 4.2 of the survey paper "What color is your Jacobian". Problem: they use an earlier star coloring algorithm so the number of colors is different.
• Add tests for banded matrices, in which case a) we know the optimal number of colors and b) we know that greedy star and acyclic colorings reach it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.21%. Comparing base (c3acf52) to head (123e9d3).

Additional details and impacted files

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##             main      #74   +/-   ##
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  Coverage   98.21%   98.21%           
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  Files          10       10           
  Lines         615      617    +2     
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+ Hits          604      606    +2     
  Misses         11       11           

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