test: fix flake in link node garbage collection test

In this commit, we fix a flake in the link node garbage collection test
by ensuring the channels have been fully closed on both sides before we
attempt to restart and ensure that they don't actually establish
connections. Without this check, it's possible that either side hasn't
yet processed all the blocks, so they'll still reconnect to each other on
start up.

lnd_test.go

@@ -5715,6 +5715,47 @@ func testGarbageCollectLinkNodes(net *lntest.NetworkHarness, t *harnessTest) {
 		t.Fatalf("unable to generate blocks: %v", err)
 	}
 
+	// Before we test reconnection, we'll ensure that the channel has been
+	// fully cleaned up for both Carol and Alice.
+	var predErr error
+	pendingChansRequest := &lnrpc.PendingChannelsRequest{}
+	err = lntest.WaitPredicate(func() bool {
+		ctxt, _ = context.WithTimeout(ctxb, timeout)
+		pendingChanResp, err := net.Alice.PendingChannels(
+			ctxt, pendingChansRequest,
+		)
+		if err != nil {
+			predErr = fmt.Errorf("unable to query for pending "+
+				"channels: %v", err)
+			return false
+		}
+
+		predErr = checkNumForceClosedChannels(pendingChanResp, 0)
+		if predErr != nil {
+			return false
+		}
+
+		ctxt, _ = context.WithTimeout(ctxb, timeout)
+		pendingChanResp, err = carol.PendingChannels(
+			ctxt, pendingChansRequest,
+		)
+		if err != nil {
+			predErr = fmt.Errorf("unable to query for pending "+
+				"channels: %v", err)
+			return false
+		}
+
+		predErr = checkNumForceClosedChannels(pendingChanResp, 0)
+		if predErr != nil {
+			return false
+		}
+
+		return true
+	}, time.Second*15)
+	if err != nil {
+		t.Fatalf("channels not marked as fully resolved: %v", predErr)
+	}
+
 	testReconnection(carol)
 
 	// Finally, we'll ensure that Bob and Carol no longer show in Alice's