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/controller/utils.go

@@ -29,3 +29,6 @@ type WithInformerOptionFunc func(
 	getter controller.InformerGetter,
 	filter controller.Filter,
 	opt controller.InformerOption) controller.Option
+
+// Same signature as controller.WithInitialEvent().
+type WithInitialEventOptionFunc func(key controller.Key) controller.Option