Update ci-testing.yml

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Update to Continuous Integration (CI) workflow, focusing only on the master branch.

📊 Key Changes

• The CI testing workflow will now only trigger on pushes to the master branch.
• Pull requests that target the master branch will also trigger the CI testing workflow.

🎯 Purpose & Impact

• 🎯 Purpose: To streamline CI processes and potentially reduce unnecessary runs/tests on branches other than master.
• 💡 Impact: Faster CI feedback for changes that are directly affecting the main codebase, while possibly saving resources and minimizing noise from development branches. Users can expect more efficient integration of new features and bug fixes.