cleanup testing

[baumgold]

1. Re-enable commented-out test from implement default for SubArray #456
2. Use separate test-specific Project.toml to allow for test-specific compat (and remove compat shim)
3. Replace testing println with testsets

[codecov-commenter]

Codecov Report

Merging #462 (e4d2146) into main (9f1d51a) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #462      +/-   ##
==========================================
- Coverage   87.53%   87.50%   -0.04%     
==========================================
  Files          26       26              
  Lines        3272     3272              
==========================================
- Hits         2864     2863       -1     
- Misses        408      409       +1     

see 1 file with indirect coverage changes

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[quinnj]

This is great; thanks @baumgold!