test: Parameterize filesystem GCS fixtures

[hashkanna]

Description of changes

Parameterize the filesystem system-test GCS fixture paths.

The filesystem data-source tests still default to gs://pso-kokoro-resources/file_connection, so Kokoro can keep using the existing fixture bucket. Local and alternate validation environments can now point the same tests at a bucket by setting TEST_BUCKET, or fall back to PROJECT_ID when the project id matches the test bucket name.

This keeps the test fixture layout unchanged while avoiding hard-coded access to the Kokoro bucket for local Ibis 7.1 validation.

Issues to be closed

N/A

Testing

• venv/bin/python -m black --check tests/system/data_sources/test_filesystem.py
• PROJECT_ID=<test-bucket-project> venv/bin/python -m pytest tests/system/data_sources/test_filesystem.py -q
• TZ=UTC venv/bin/python -m pytest tests/unit -q
• git diff --check upstream/ibis-7.1-modernization...HEAD

Checklist

• I have followed the CONTRIBUTING Guide.
• I have commented my code, particularly in hard-to-understand areas
• I have updated any relevant documentation to reflect my changes, if applicable
• I have added unit and/or integration tests relevant to my change as needed
• I have already checked locally that all unit tests and linting are passing (use the tests/local_check.sh script)
• I have manually executed end-to-end testing (E2E) with the affected databases/engines