test: use defineConfig in vitest config

[yamachi4416]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Changed to defineConfig as test-utils's defineVitestProject is intended for nuxt client-environments.

Please refer to: nuxt/nuxt#34645

[what-the-diff]

PR Summary

• Import Statement Update
  The code has been changed to import configurations from 'vitest/config' replacing the previous source '@nuxt/test-utils/config'. This implies that the new configurations will be fetched from 'vitest/config', which may offer better or updated features and functionalities.
• Configuration Function Modification
  The function for defining configurations in the code has been renamed to 'defineConfig' from its previous name 'defineVitestConfig'. This change might have been implemented for better code readability, understandability or due to alterations in the software's design paradigm.