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

Attention: Patch coverage is 96.55172% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.92%. Comparing base (3438e16) to head (774bade).

Files with missing lines	Patch %	Lines
numcodecs/tests/test_checksum32.py	96.55%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (96.55%) is below the target coverage (100.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #740      +/-   ##
==========================================
- Coverage   99.96%   99.92%   -0.04%     
==========================================
  Files          63       63              
  Lines        2736     2736              
==========================================
- Hits         2735     2734       -1     
- Misses          1        2       +1     

Files with missing lines	Coverage Δ
numcodecs/tests/test_checksum32.py	98.82% <96.55%> (-1.18%)	⬇️

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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.