tstest/natlab/vmtest: add test for direct conn with cached netmap (#19660)

When a peer is not able to connect to control after a restart and is
using a cached netmap, that nodes should be able to connect to another
peer in its tailnet (given that the home DERP of that peer has not
changed in the meantime).

Add test that starts two peers and connects them to a tailnet with
caching enabled. Then blackhole traffic to control from one peer and
restart it. Verify that the connection between the two ends up direct.

Adds facilities for expecting a certain path type between nodes.

Updates: #19597

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

tstest/natlab/vnet/conf.go

@@ -445,6 +445,12 @@ func (n *Network) PostConnectedToControl() {
 	n.network.SetControlBlackholed(n.postConnectBlackholeControl)
 }
 
+// BlackholeControlForAddr sets weither the network should drop all control
+// traffic for the specified addr starting immediately.
+func (n *Network) BlackholeControlForAddr(addr netip.Addr) {
+	n.network.BlackholeControlForAddr(addr)
+}
+
 // NetworkService is a service that can be added to a network.
 type NetworkService string