docs: Enhance TESTING.md with comprehensive testing guidelines and be…

[VolodymyrBg]

…st practices

This PR significantly improves the TESTING.md documentation by adding detailed information about:

• Testing framework and tools used (Hardhat, Mocha, Chai)
• Instructions for running tests and generating coverage reports
• Test structure and organization conventions
• Best practices for writing effective tests
• Testing fixtures and utilities available in the repository
• Continuous integration setup
• Guidelines for contributing tests
• Advanced testing techniques including security-specific approaches

This enhancement provides valuable guidance for contributors, making it easier for them to understand the testing expectations and standards of the OpenZeppelin Contracts project. The improved documentation also aligns with the project's commitment to high-quality, secure smart contracts by emphasizing thorough testing practices.

Fixes #????

PR Checklist

• Tests
• Documentation
• Changeset entry (run npx changeset add)