Add Hub custom models to CI tests

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhanced CI testing workflow for the YOLOv5 repository.

📊 Key Changes

• Standardized the formatting of YAML arrays in the workflow file for clarity.
• Expanded the testing script to include additional model verification steps after training and detection.

🎯 Purpose & Impact

• Improved Readability: Uniform spaces make the workflow file more readable and easier to maintain.
• Rigorous Testing: Incorporating more comprehensive tests, including model exporting and loading, increases the confidence in model performance and stability across different platforms.
• Enhanced Robustness: Regular and thorough testing ensures that any potential integration issues are caught early, maintaining the reliability of the YOLOv5 models for users.