Move generated *.json annotations into runs/test

[glenn-jocher]

This PR moves prediction annotation json files into new runs/test directory on pycocotools testing.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Improved JSON file handling and naming in YOLOv5 testing script.

📊 Key Changes

• Changed how the JSON detection results file is named when saving.
• Updated the file handling to accommodate different types (list, str, None) of the weights parameter.
• Simplified the result file path construction using Path from pathlib.
• Ensured the loadRes method receives the proper string file path.

🎯 Purpose & Impact

• 🎨 Enhances consistency in file naming for easier identification and organization of output files.
• 🛠 Fixes potential issues with file paths when weights parameter varies in type, improving robustness.
• 📈 May facilitate better integration with other tools and post-processing due to standardized file naming.
• 👥 Affects users who save JSON detections, making their workflow smoother and their outputs more predictable.