Add test coverage for `ClosureReason::DisconnectedPeer`

[valentinewallace]

When we're opening a channel to a peer, if the peer disconnects before the funding transaction is broadcasted, then the channel is force-closed with ClosureReason::DisconnectedPeer. We're currently missing test coverage for this case

[ViktorTigerstrom]

I could look into this one if no one else is working on this :)