Test that dual-funded channel opens error for existing channel ID

[dunxen]

We need an equivalent of the following test for dual-funded channel opens:

assert!(err_msg.contains("An existing channel using ID"));
assert!(err_msg.contains("is open with peer"));
let channel_id = ChannelId::v1_from_funding_outpoint(funding_output);
let reason = ClosureReason::ProcessingError { err: format!("An existing channel using ID {} is open with peer {}", channel_id, nodes[1].node.get_our_node_id()), };

See context: #3638