If the `<Transition />` component "root" is used as a root placeholder
(for state management) and not making actual transitions itself, then we
don't require a `ref` element.

+ add `className="…"` to some places to indicate that we _do_ want to
  perform a transition and thus have to fail if the `ref` is missing.

Also ensure that a ref is required if the `as` prop is provided and
it's not a `Fragment`

These tests were rendering a `Debug` element that didn't render any DOM
nodes. Adding `as="div"` ensures that we are forwarding the ref
correctly.