Run tests concurrently in CI

[Yonghui-Lee]

This PR enables concurrent test execution in CI using pytest-xdist to speed up the build process. It also addresses several test failures that were exposed by running against the emulator in a parallel environment.

Key Changes

CI Infrastructure & Parallelization

• Enabled pytest-xdist: Added -n auto to pytest calls in .github/workflows/ci.yml and added pytest-xdist to environment_gcsfs.yaml and pyproject.toml.
• Isolated Worker Buckets: Modified gcsfs/tests/settings.py to append the xdist worker ID (e.g., _gw0) to bucket names. This ensures that parallel workers do not conflict by operating on the same buckets.
• Global Emulator Setup: Moved the GCS emulator startup to a dedicated step in ci.yml and set STORAGE_EMULATOR_HOST globally. This ensures all workers share a single emulator instance rather than trying to spin up conflicting docker containers.

Test Fixes & Cleanups

• Skipped test_sign on Emulator: Skipped this test entirely when running against the emulator. The emulator does not support signed URL validation.
• Skipped test_credentials_from_raw_token on Emulator: Skipped this test on the emulator because the emulator accepts any token, causing the test to fail when it expects an invalid credentials error.
• Consolidated Dummy Credentials: Deleted gcsfs/tests/fake-secret.json which was malformed (invalid PEM format) and caused DefaultCredentialsError in extended tests. Updated CI to use fake-service-account-credentials.json which contains a validly formatted dummy key.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.19%. Comparing base (991faba) to head (36e6fe2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #845      +/-   ##
==========================================
- Coverage   88.52%   88.19%   -0.34%     
==========================================
  Files          15       15              
  Lines        2989     2989              
==========================================
- Hits         2646     2636      -10     
- Misses        343      353      +10     

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