vnet: add control/derps to test, stateful firewall

Updates #13038

Change-Id: Icd65b34c5f03498b5a7109785bb44692bce8911a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/natlab/vnet/conf.go

@@ -27,6 +27,10 @@ type Config struct {
 	networks []*Network
 }
 
+func (c *Config) NumNodes() int {
+	return len(c.nodes)
+}
+
 // AddNode creates a new node in the world.
 //
 // The opts may be of the following types:
@@ -110,6 +114,11 @@ type Node struct {
 	nets []*Network
 }
 
+// MAC returns the MAC address of the node.
+func (n *Node) MAC() MAC {
+	return n.mac
+}
+
 // Network returns the first network this node is connected to,
 // or nil if none.
 func (n *Node) Network() *Network {