改进测试套件文档

1. Create tests/README.md

Add comprehensive test documentation including:

• Overview of test structure and organization
• How to run all tests: pytest or Python -m pytest
• How to run specific test files: pytest tests/test_cacheutils.py
• How to run specific test functions: pytest tests/test_cacheutils.py::test_lru_add
• Test naming conventions
• Explanation of test data files (jsonl_test_data.txt, newlines_test_data.txt)
• Coverage reporting instructions
• CI/CD integration details