Junit for Consumer class returning lambda

I am writing junit test case for below for consumer class which has below method.

@Bean @Transactional public Consumer<Message<String>> response() {      return message -> {         Acknowledgment ack = message.getHeaders().get(Ack, Acknowledgment.class);        helper(message);         log.info("hello");         ack.acknowlege();     }

I am trying to go inside the lambda function through test method but able to proceed further. I tried to write the test as below:

@Testpublic void testHelper() {    Message<String> message = MessageBuilder.withPayload("Hello").build();    consumer.response();}