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>

util/eventbus/eventbustest/doc.go

@@ -39,6 +39,20 @@
 // checks that the stream contains exactly the given events in the given order,
 // and no others.
 //
+// To test for the absence of events, use [ExpectExactly] without any
+// expected events, along side [testing/synctest] to avoid waiting for timers
+// to ensure that no events are produced. This will look like:
+//
+//	synctest.Test(t, func(t *testing.T) {
+//		bus := eventbustest.NewBus(t)
+//		tw := eventbustest.NewWatcher(t, bus)
+//		somethingThatShouldNotEmitsSomeEvent()
+//		synctest.Wait()
+//		if err := eventbustest.ExpectExactly(tw); err != nil {
+//			t.Errorf("Expected no events or errors, got %v", err)
+//		}
+//	})
+//
 // See the [usage examples].
 //
 // [usage examples]: https://github.com/tailscale/tailscale/blob/main/util/eventbus/eventbustest/examples_test.go