chore: test correctness fixes

[d-v-b]

claude-generated fixes of claude-discovered issues in our test suite

Downstream gpu marker was never registered

src/zarr/testing/conftest.py defined pytest_configure to register the gpu marker, but the pytest11 entry point is zarr.testing (src/zarr/testing/__init__.py), which has no pytest_configure — so downstream projects consuming StoreTests under --strict-markers never got the marker.

• Add pytest_configure to src/zarr/testing/__init__.py (registers gpu, comment kept).
• Delete the now-dead src/zarr/testing/conftest.py and its [tool.coverage.run] omit entry.
• Import assert_bytes_equal only when pytest is available (it imports pytest), matching StoreTests.

zarr-python's own suite still registers gpu via the main markers list, so local behavior is unchanged. Verified the marker now appears via the plugin in an isolated empty directory.

Pin real exception messages on broad pytest.raises

Added verified match= regexes to ~16 high-value broad-builtin pytest.raises in tests/test_api.py, tests/test_group.py, and tests/test_store/test_core.py. Every message was confirmed against the actual code, not guessed.

Remove silently no-op assertions

In tests/test_experimental/test_cache_store.py, dropped the if hasattr(cached_store, "_is_key_fresh") guards (the method exists on CacheStore) so the TTL/freshness checks always run.

xfail/skip hygiene

Added reason= to two reason-less xfail params in tests/test_indexing.py. The env-specific unconditional skip on test_setitem_repeated_index was left as-is (repeated-index assignment has undefined ordering and fails broadly, so no correct skipif exists).

Verification

• pytest tests/test_api.py tests/test_group.py tests/test_store/test_core.py tests/test_experimental/test_cache_store.py tests/test_indexing.py: 1413 passed, 25 skipped, 5 xfailed.
• Full-suite collection: 6479 tests, no errors. ruff/mypy pre-commit pass.

Part of #195

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.50%. Comparing base (a63ebd5) to head (68ec724).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4094   +/-   ##
=======================================
  Coverage   93.50%   93.50%           
=======================================
  Files          90       90           
  Lines       11979    11982    +3     
=======================================
+ Hits        11201    11204    +3     
  Misses        778      778           

Files with missing lines	Coverage Δ
src/zarr/testing/__init__.py	90.90% <100.00%> (+3.40%)	⬆️

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