multi: link quit can interrupt commitment signing

In this commit, we make sig job handling when singing a next commitment
non-blocking by allowing the shutdown of a channel link to prevent
further waiting on sig jobs by the channel state machine. This addresses
possible cases where the aux signer may be shut down via a separate quit
signal, so the state machine could block indefinitely on receiving an
update on a sig job.

htlcswitch/test_utils.go

@@ -1197,12 +1197,14 @@ func (h *hopNetwork) createChannelLink(server, peer *mockServer,
 	}
 
 	go func() {
-		for {
-			select {
-			case <-notifyUpdateChan:
-			case <-link.(*channelLink).quit:
-				close(doneChan)
-				return
+		if chanLink, ok := link.(*channelLink); ok {
+			for {
+				select {
+				case <-notifyUpdateChan:
+				case <-chanLink.Quit:
+					close(doneChan)
+					return
+				}
 			}
 		}
 	}()