Add tests for structured matrices

[gdalle]

Add some tests (no custom implementations yet) for the following structured matrix types.

Package	Type	Coloring	Decompression	Performance
LinearAlgebra	Diagonal	✅	✅	✅
LinearAlgebra	Bidiagonal	✅	✅	✅
LinearAlgebra	Tridiagonal	✅	✅	✅
BandedMatrices	BandedMatrix	✅	✅
BlockBandedMatrices	BlockBandedMatrix	✅	✅
BlockBandedMatrices	BandedBlockBandedMatrix	✅	✅

The fallback decompression already works without error: it follows the sparsity pattern, so it avoids writing into forbidden locations (say, outside of the diagonal for a Diagonal).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (1cb3884) to head (61da959).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #137   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           11        11           
  Lines          871       871           
=========================================
  Hits           871       871           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.