test: fixed the 'Data exists' error that occurred when test case executions overlapped

[sttk]

Since a 'Data exists' error occurred when test cases ran concurrently, attempting to operate on the same key in the same database, I changed it so that each test case runs with a different database number.

[Copilot]

Pull Request Overview

This PR addresses concurrency issues in Redis-based test cases by ensuring each test uses a different Redis database number to prevent "Data exists" errors when tests run simultaneously.

• Assigns unique Redis database numbers (0, 1, 2, 3) to different test functions
• Converts helper functions to standalone test functions with proper #[test] attributes
• Removes the combined test function that was calling multiple test scenarios sequentially

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