client-go informers: replace time.Sleep with callback

While time.Sleep is what the test needs, maybe an arbitrary hook invocation is
more acceptable in the production code because it is more general.

Kubernetes-commit: 2ec0305d728bf5ce8f8df314a18e71aa120a00cf

tools/cache/shared_informer.go

@@ -895,15 +895,15 @@ func (p *sharedProcessor) distribute(obj interface{}, sync bool) {
 	}
 }
 
-// sharedProcessorRunDelay is used in a synctest bubble to achieve a certain ordering of
-// steps in different goroutines.
-var sharedProcessorRunDelay atomic.Pointer[time.Duration]
+// sharedProcessorRunHook can be used inside tests to execute additional code
+// at the start of sharedProcessor.run.
+var sharedProcessorRunHook atomic.Pointer[func()]
 
 func (p *sharedProcessor) run(ctx context.Context) {
 	func() {
-		delay := sharedProcessorRunDelay.Load()
-		if delay != nil {
-			time.Sleep(*delay)
+		hook := sharedProcessorRunHook.Load()
+		if hook != nil {
+			(*hook)()
 		}
 		// Changing listenersStarted needs a write lock.
 		p.listenersLock.Lock()

tools/cache/shared_informer_test.go

@@ -245,8 +245,13 @@ func testListenerResyncPeriods(t *testing.T, startupDelay time.Duration) {
 		t.Logf("%s: %s", delta, msg)
 	}
 
-	sharedProcessorRunDelay.Store(&startupDelay)
-	defer sharedProcessorRunDelay.Store(nil)
+	if startupDelay > 0 {
+		hook := func() {
+			time.Sleep(startupDelay)
+		}
+		sharedProcessorRunHook.Store(&hook)
+		defer sharedProcessorRunHook.Store(nil)
+	}
 
 	// source simulates an apiserver object endpoint.
 	source := newFakeControllerSource(t)