util/eventbus/eventbustest: add support for synctest instead of timers (#17522)

Before synctest, timers was needed to allow the events to flow into the
test bus. There is still a timer, but this one is not derived from the
test deadline and it is mostly arbitrary as synctest will render it
practically non-existent.

With this approach, tests that do not need to test for the absence of
events do not rely on synctest.

Updates #15160

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

net/netmon/netmon_test.go

@@ -144,7 +144,7 @@ func TestMonitorMode(t *testing.T) {
 		<-done
 		t.Logf("%v callbacks", n)
 	case "eventbus":
-		tw.TimeOut = *monitorDuration
+		time.AfterFunc(*monitorDuration, bus.Close)
 		n := 0
 		mon.Start()
 		eventbustest.Expect(tw, func(event *ChangeDelta) (bool, error) {