[BlockSparseArrays] Tests for slicing

[mtfishman]

Followup to #1332 to add more tests for functionality implemented there.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.68%. Comparing base (957f2af) to head (d0f5bc9).

❗ Current head d0f5bc9 differs from pull request most recent head 4a8b373. Consider uploading reports for the commit 4a8b373 to get more accurate results

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Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1360       +/-   ##
===========================================
- Coverage   84.40%   53.68%   -30.73%     
===========================================
  Files         100       99        -1     
  Lines        8581     8528       -53     
===========================================
- Hits         7243     4578     -2665     
- Misses       1338     3950     +2612     

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