discovery/gossiper: send node anns when constructing full chan proof

In this commit, we allow the gossiper to also broadcast the
corresponding node announcements, if we know of them, of a channel when
constructing its full proof. We do this to ensure peers (other than our
remote peer) receive all the relevant announcements for a channel.

The tests changes were made to ensure the new behavior introduced works
as intended. Previously, the node announcements for each test channel
announcement were not processed, so they never existed from the
gossiper's point of view.

This also addresses an existing flake in the integration test
`testNodeAnnouncement`. This problem arose due to the node announcement
being sent before the connection between Dave (node announcement sender)
and Alice (node announcement receiver) was initiated and the full
channel proof was constructed.

discovery/gossiper.go

@@ -2366,6 +2366,31 @@ func (d *AuthenticatedGossiper) processNetworkAnnouncement(
 			})
 		}
 
+		// We'll also send along the node announcements for each channel
+		// participant if we know of them.
+		node1Ann, err := d.fetchNodeAnn(chanInfo.NodeKey1Bytes)
+		if err != nil {
+			log.Debugf("Unable to fetch node announcement for "+
+				"%x: %v", chanInfo.NodeKey1Bytes, err)
+		} else {
+			announcements = append(announcements, networkMsg{
+				peer:   nMsg.peer,
+				source: nMsg.source,
+				msg:    node1Ann,
+			})
+		}
+		node2Ann, err := d.fetchNodeAnn(chanInfo.NodeKey2Bytes)
+		if err != nil {
+			log.Debugf("Unable to fetch node announcement for "+
+				"%x: %v", chanInfo.NodeKey2Bytes, err)
+		} else {
+			announcements = append(announcements, networkMsg{
+				peer:   nMsg.peer,
+				source: nMsg.source,
+				msg:    node2Ann,
+			})
+		}
+
 		nMsg.err <- nil
 		return announcements
 
@@ -2375,6 +2400,19 @@ func (d *AuthenticatedGossiper) processNetworkAnnouncement(
 	}
 }
 
+// fetchNodeAnn fetches the latest signed node announcement from our point of
+// view for the node with the given public key.
+func (d *AuthenticatedGossiper) fetchNodeAnn(
+	pubKey [33]byte) (*lnwire.NodeAnnouncement, error) {
+
+	node, err := d.cfg.Router.FetchLightningNode(pubKey)
+	if err != nil {
+		return nil, err
+	}
+
+	return node.NodeAnnouncement(true)
+}
+
 // sendAnnSigReliably will try to send the provided local AnnounceSignatures
 // to the remote peer, waiting for it to come online if necessary. This
 // method returns after adding the message to persistent storage, such