Add integration and more unit tests

- Add integration test for serving cert auto-generation and rotation
- Add unit test for `WithInitialEvent` of the cert manager controller
- Move UpdateAPIService() into the `apicerts` package, since that is
  the only user of the function.

internal/controllermanager/prepare_controllers.go

@@ -66,6 +66,7 @@ func PrepareControllers(
 				aggregatorClient,
 				installationNamespaceK8sInformers.Core().V1().Secrets(),
 				controller.WithInformer,
+				controller.WithInitialEvent,
 			),
 			singletonWorker,
 		).