multi: Add tests to psbt and normal open channel flow.

Itests were added to the normal channel funding flow and the psbt
funding flow using unstable (unconfirmed sweeper inputs).

lntest/harness.go

@@ -1205,6 +1205,7 @@ func (h *HarnessTest) OpenChannelAssertErr(srcNode, destNode *node.HarnessNode,
 
 	// Receive an error to be sent from the stream.
 	_, err := h.receiveOpenChannelUpdate(respStream)
+	require.NotNil(h, err, "expected channel opening to fail")
 
 	// Use string comparison here as we haven't codified all the RPC errors
 	// yet.