lntest: Add ShutdownNode method to harness.

This is preferable to calling Shutdown on the node directly so that
the harness manages the entire lifecycle of an lnd process.

lntest/harness.go

@@ -224,7 +224,7 @@ out:
 // TearDownAll tears down all active nodes within the test lightning network.
 func (n *NetworkHarness) TearDownAll() error {
 	for _, node := range n.activeNodes {
-		if err := node.Shutdown(); err != nil {
+		if err := n.ShutdownNode(node); err != nil {
 			return err
 		}
 	}
@@ -341,6 +341,17 @@ func (n *NetworkHarness) RestartNode(node *HarnessNode, callback func() error) e
 	return node.restart(n.lndErrorChan, callback)
 }
 
+// ShutdownNode stops an active lnd process and returns when the process has
+// exited and any temporary directories have been cleaned up.
+func (n *NetworkHarness) ShutdownNode(node *HarnessNode) error {
+	if err := node.shutdown(); err != nil {
+		return err
+	}
+
+	delete(n.activeNodes, node.NodeID)
+	return nil
+}
+
 // TODO(roasbeef): add a WithChannel higher-order function?
 //  * python-like context manager w.r.t using a channel within a test
 //  * possibly  adds more funds to the target wallet if the funds are not