Use pytest.skipif to properly report skipped CRC32C codec tests

[agriyakhetarpal]

The tests test_err_location, test_repr, and test_backwards_compatibility were not reporting that they were skipped if the crc32c dependency wasn't available/installed, as they weren't marked with the appropriate markers. This PR skips these tests at the pytest level rather than using a has_crc32c check that defines the tests at collection. I've parametrized most of these tests. I had to split up the test_backwards_compatibility test into two, as CRC32C() wouldn't be defined if it were not installed (I don't think it makes sense to mock it just for one test).

Closes #640

TODO:

• Unit tests and/or doctests in docstrings
• Tests pass locally
• Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.rst
• Docs build locally
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

[codecov]

Codecov Report

❌ Patch coverage is 86.20690% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.08%. Comparing base (fedb0cb) to head (1e394e7).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
numcodecs/tests/test_checksum32.py	86.20%	4 Missing ⚠️

❌ Your patch status has failed because the patch coverage (86.20%) is below the target coverage (100.00%). You can increase the patch coverage or adjust the target coverage.
❌ Your project status has failed because the head coverage (89.08%) is below the target coverage (100.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #740      +/-   ##
==========================================
+ Coverage   89.00%   89.08%   +0.07%     
==========================================
  Files          64       64              
  Lines        2574     2574              
==========================================
+ Hits         2291     2293       +2     
+ Misses        283      281       -2     

Files with missing lines	Coverage Δ
numcodecs/tests/test_checksum32.py	84.04% <86.20%> (+2.12%)	⬆️

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[agriyakhetarpal]

🤔 I'm not sure if the Codecov report makes sense here. I've just changed test_err_location to use if not has_crc32c and skip it instead of using if has_crc32c. It should mean the same thing.

[agriyakhetarpal]

Thanks! I think the CodeCov report is spurious; these are test files and aren't missing coverage. I suppose they should be excluded.

[d-v-b]

yes i'm happy ignoring codecov here and fixing it in a separate effort. Thanks for this PR!