Avoid WUXC folders in Runtime Tests

### What would you like to be added? 💡

Previously we had to separate Windows.UI.Xaml.Controls-based controls from Microsoft.UI.Xaml.Controls ones, but now this is no longer necessary as UWP support was dropped.

### Why is this needed? 🤔

_No response_

### Renderer 🎨

- [ ] Skia
- [ ] Native

### For which platforms 📱💻🖥️

_No response_

### Anything else we need to know? 💬

_No response_