tstest/natlab: add missing tests from earlier commits

Now you can actually see that packet delivery works.

Pairing with @danderson

tstest/natlab/natlab_test.go

@@ -0,0 +1,81 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package natlab
+
+import (
+	"fmt"
+	"testing"
+
+	"inet.af/netaddr"
+)
+
+func TestAllocIPs(t *testing.T) {
+	n := NewInternet()
+	saw := map[netaddr.IP]bool{}
+	for i := 0; i < 255; i++ {
+		for _, f := range []func(*Machine) netaddr.IP{n.allocIPv4, n.allocIPv6} {
+			ip := f(nil)
+			if saw[ip] {
+				t.Fatalf("got duplicate %v", ip)
+			}
+			saw[ip] = true
+		}
+	}
+
+	// This should work:
+	n.allocIPv6(nil)
+
+	// But allocating another IPv4 should panic, exhausting the
+	// limited /24 range:
+	defer func() {
+		if e := recover(); fmt.Sprint(e) != "pool exhausted" {
+			t.Errorf("unexpected panic: %v", e)
+		}
+	}()
+	n.allocIPv4(nil)
+	t.Fatalf("expected panic from IPv4")
+}
+
+func TestSendPacket(t *testing.T) {
+	internet := NewInternet()
+
+	foo := NewMachine("foo")
+	bar := NewMachine("bar")
+	ifFoo := foo.Attach("eth0", internet)
+	ifBar := bar.Attach("enp0s1", internet)
+
+	t.Logf("foo IP: %v, %v", ifFoo.V4(), ifFoo.V6())
+	t.Logf("bar IP: %v, %v", ifBar.V4(), ifBar.V6())
+
+	fooAddr := netaddr.IPPort{IP: ifFoo.V4(), Port: 123}
+	barAddr := netaddr.IPPort{IP: ifBar.V4(), Port: 456}
+
+	fooPC, err := foo.ListenPacket("udp4", fooAddr.String())
+	if err != nil {
+		t.Fatal(err)
+	}
+	barPC, err := bar.ListenPacket("udp4", barAddr.String())
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	const msg = "some message"
+	if _, err := fooPC.WriteTo([]byte(msg), barAddr.UDPAddr()); err != nil {
+		t.Fatal(err)
+	}
+
+	buf := make([]byte, 1500) // TODO: care about MTUs in the natlab package somewhere
+	n, addr, err := barPC.ReadFrom(buf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	buf = buf[:n]
+	if string(buf) != msg {
+		t.Errorf("read %q; want %q", buf, msg)
+	}
+	if addr.String() != fooAddr.String() {
+		t.Errorf("addr = %q; want %q", addr, fooAddr)
+	}
+}