health: remove direct callback and replace with eventbus (#17199)

Pulls out the last callback logic and ensures timers are still running.

The eventbustest package is updated support the absence of events.

Updates #15160

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

util/eventbus/eventbustest/eventbustest.go

@@ -120,7 +120,12 @@ func Expect(tw *Watcher, filters ...any) error {
 // [Expect]. Use [Expect] if other events are allowed.
 func ExpectExactly(tw *Watcher, filters ...any) error {
 	if len(filters) == 0 {
-		return errors.New("no event filters were provided")
+		select {
+		case event := <-tw.events:
+			return fmt.Errorf("saw event type %s, expected none", reflect.TypeOf(event))
+		case <-time.After(tw.TimeOut):
+			return nil
+		}
 	}
 	eventCount := 0
 	for pos, next := range filters {

util/eventbus/eventbustest/eventbustest_test.go

@@ -250,7 +250,7 @@ func TestExpectEvents(t *testing.T) {
 			tw := eventbustest.NewWatcher(t, bus)
 			// TODO(cmol): When synctest is out of experimental, use that instead:
 			// https://go.dev/blog/synctest
-			tw.TimeOut = 10 * time.Millisecond
+			tw.TimeOut = 100 * time.Millisecond
 
 			client := bus.Client("testClient")
 			defer client.Close()