Wait peers to connect before doing anything else (#7375)

There is a race condition in `NetworkBridgeRx` between registering
incoming peers and handling overseer signals. While the peer connection
was sent to `NetworkBridgeRx` first, sometimes the peer would be added to
`validation/collation_peers` with enough delay that the bridge handled
an overseer signal that was meant do send notifications to the registered
peer and as the peer was not present in the bridge, the notification was
never sent and the test would hang.

When peers are registered to `NetworkBridgeRx` using `connect_peer()`,
wait until they show up in `shared.validation/collation_peers` before
doing anything else.

Co-authored-by: parity-processbot <>