[slider] Targeting the Thumb properly in a Jest/Vitest environment

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Developers using our Base UI based design system were confused as to why in order to select a `Slider.Thumb` in a test, one must use `{ hidden: true }`:
```
expect(screen.getByRole('slider', { hidden: true })).toBeInTheDocument()
```

I noticed these lines must be the cause: https://github.com/mui/base-ui/blob/master/packages/react/src/slider/thumb/SliderThumb.tsx#L251-L254

```
return {
['--position' as string]: `${positionPercent}%`,
visibility:
(renderBeforeHydration && !isMounted) || positionPercent === undefined
? 'hidden'
: undefined,
```

I can't explain why this condition exists and if there is a way around this to keep our tests clean (and avoid using `{ hidden: true }` ?