Make it easier to test only test shadow DOM trees

With PR https://github.com/dequelabs/axe-core/pull/3798 it has become possible to include elements inside shadow DOM. This then automatically includes all slotted content too. It would be nice if there was an easy way to only test the shadow DOM tree, for example:

```js
axe.run({
include: { fromShadowDom: ['#shadowDom', ':root'],
exclude: { fromShadowDom: ['#shadowDom', 'slot']
});
```