Poll the substream validation before polling `Notifications` (#13934)

* Poll the substream validation before polling `Notifications`

In tests, it can happen that `Notifications` doesn't produce any events
which causes `poll()` to return `Poll::Pending` and the substream
validation futures won't get polled.

Poll the futures before calling `Notifications` so results for substream
validations are received even if `Notifications` is not producing any
events.

* Remove `pending_messages`

* Remove unused import