wgengine/monitor: skip some macOS route updates, fix debounce regression

Debound was broken way back in 5c1e443d34 and we never noticed.

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

wgengine/monitor/monitor_test.go

@@ -5,7 +5,10 @@
 package monitor
 
 import (
+	"flag"
 	"testing"
+
+	"tailscale.com/net/interfaces"
 )
 
 func TestMonitorStartClose(t *testing.T) {
@@ -28,3 +31,35 @@ func TestMonitorJustClose(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+var monitor = flag.String("monitor", "", `go into monitor mode like 'route monitor'; test never terminates. Value can be either "raw" or "callback"`)
+
+func TestMonitorMode(t *testing.T) {
+	switch *monitor {
+	case "":
+		t.Skip("skipping non-test without --monitor")
+	case "raw", "callback":
+	default:
+		t.Skipf(`invalid --monitor value: must be "raw" or "callback"`)
+	}
+	mon, err := New(t.Logf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	switch *monitor {
+	case "raw":
+		for {
+			msg, err := mon.om.Receive()
+			if err != nil {
+				t.Fatal(err)
+			}
+			t.Logf("msg: %#v", msg)
+		}
+	case "callback":
+		mon.RegisterChangeCallback(func(changed bool, st *interfaces.State) {
+			t.Logf("cb: changed=%v, ifSt=%v", changed, st)
+		})
+		mon.Start()
+		select {}
+	}
+}