[select] Define which children are vs are not slotted.

These two things rendering differently in Chrome makes zero sense to me, but WPT kind of expects it:

```html
<!doctype html>
<select multiple>abc</select>
<select multiple><span>abc</span></select>
```

cc @josepharhar am I missing something? I plan to otherwise update the expectations to render the text in both cases.

cc @keithamus @jnjaeschke @annevk 