docs: Add comprehensive Mock Server testing documentation

- Created new TESTING_CONNECTORS.md guide covering HttpMocker utilities
- Included quick start examples and core component documentation
- Added testing patterns for full refresh, incremental, pagination, and error handling
- Documented advanced techniques like request builders and time freezing
- Included real-world examples from Salesforce and Hubspot connectors
- Added best practices and troubleshooting sections

Co-Authored-By: AJ Steers <aj@airbyte.io>

Author: devin-ai-integration[bot]