tstest/natlab/vmtest: add more subnet router tests

Add two tests building on TestExitNode's framework:

TestSubnetRouterPublicIP brings up a client, a subnet router, and a
webserver, each on its own NAT'd network with distinct WAN IPs. The
subnet router advertises the webserver's network as a route. The test
toggles the client's --accept-routes preference and asserts that the
webserver's echoed source IP switches between the client's own WAN
(direct dial) and the subnet router's WAN (forwarded through the
router and SNAT'd).

TestSubnetRouterAndExitNode adds a fourth node, an exit node that
advertises 0.0.0.0/0 + ::/0, and uses a table-driven layout with
subtests to cover the four combinations of (exit on/off, subnet
on/off). The case where both are on confirms longest-prefix match
wins: the subnet router's /24 takes precedence over the exit node's
/0. The exit node itself is configured with --accept-routes=off so
that, in the exit-only case, it forwards directly to the simulated
internet rather than re-routing the forwarded traffic via the subnet
router (which would otherwise mask the exit node's WAN as the
observed source).

Adds an Env.SetAcceptRoutes helper for toggling the RouteAll pref via
EditPrefs, used by both tests.

Updates #13038

Change-Id: Ifc2726db1df2f039c477c222484f535bebc40445
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/natlab/vmtest/vmtest.go

@@ -426,6 +426,23 @@ func (e *Env) SetExitNode(client, exitNode *Node) {
 	}
 }
 
+// SetAcceptRoutes toggles the node's RouteAll preference (the
+// --accept-routes flag), controlling whether it installs subnet routes
+// advertised by peers.
+func (e *Env) SetAcceptRoutes(n *Node, on bool) {
+	e.t.Helper()
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	if _, err := n.agent.EditPrefs(ctx, &ipn.MaskedPrefs{
+		Prefs:       ipn.Prefs{RouteAll: on},
+		RouteAllSet: true,
+	}); err != nil {
+		e.t.Fatalf("SetAcceptRoutes(%s, %v): %v", n.name, on, err)
+	}
+	e.t.Logf("[%s] accept-routes=%v", n.name, on)
+}
+
 // ApproveRoutes tells the test control server to approve subnet routes
 // for the given node. The routes should be CIDR strings.
 func (e *Env) ApproveRoutes(n *Node, routes ...string) {