chore: fix lint failures in spanner samples

[sakthivelmanii]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request performs minor cleanup across several test files in the Google Cloud Spanner samples. Specifically, it removes unused pytest imports in autocommit_test.py and quickstart_test.py, and adjusts whitespace for better PEP 8 compliance in backup_snippet_test.py, conftest.py, and graph_snippets_test.py. I have no feedback to provide.