Add Torch matrix test + improve sparse matrix hypothesis infra

[mdekstrand]

This improves the test utilities for generating sparse matrices for testing, and adds tests for PyTorch sparse matrix logic to ensure that our underlying compute substrate is well-behaved for algorithm implementation.

[mdekstrand]

This is testing PyTorch bug pytorch/pytorch#127491, which is exercised in isolation in https://github.com/mdekstrand/torch-spmv-bug-repro.

[codecov]

Codecov Report

Attention: Patch coverage is 86.04651% with 6 lines in your changes missing coverage. Please review.

Project coverage is 86.27%. Comparing base (81dfaff) to head (8ffed10).
Report is 791 commits behind head on main.

Files with missing lines	Patch %	Lines
lenskit/util/test.py	84.61%	4 Missing ⚠️
lenskit/data/matrix.py	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #408      +/-   ##
==========================================
- Coverage   86.57%   86.27%   -0.31%     
==========================================
  Files          48       48              
  Lines        3188     3212      +24     
==========================================
+ Hits         2760     2771      +11     
- Misses        428      441      +13     

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