Testing spring-integration flow using Http.outboundGateway() with configured RestTemplate

I have a situation where I'm writing unit tests for an IntegrationFlow that relies on a configure RestTemplate that does authentication.

@Configuration
public class XXIntegrationConfig {
    @Autowired
    private RestTemplate restTemplate;

    @Bean
    public IntegrationFlow readRemote() {
        return f ->
            f.handle(
                Http.outboundGateway("http://localhost:8080/main/{mainId}", restTemplate)
                    .httpMethod(HttpMethod.GET)
                    .uriVariable("mainId", "headers.mainId")
                    .expectedResponseType(String.class)
            )
            .transformers(Transformers.fromJson())
            .enrich(enrichSecondary())
            .transform(Transformers.toJson());
    }

    @Bean
    public Consumer<EnricherSpec> enrichSecondary() {
        return e ->
            e.requestSubFlow(
                esf -> esf.handle(
                    Http.outboundGateway("http://localhost:8080/main/{mainId}/secondary", restTemplate)
                        .httpMethod(HttpMethod.GET)
                        .uriVariable("mainId", "headers.mainId")
                        .mappedResponseHeaders()
                        .expectedResponseType(String.class)
                )
                .transform(Transformers.fromJson())
            )
            .propertyExpression("secondary", "payload.value");
    }
}

I am having difficulty establishing the test where the restTemplate that is @Autowired is a Mock.

I have tried something similar to the following with no success

@SpringBootTest
@SpringIntegrationTest
public class XXIntegrationConfigTests {
    @Mock
    private RestTemplate restTemplate;

    @InjectMocks
    @Autowired
    private XXIntegrationConfig xxIntegrationConfig;

    @Autowired
    private IntegrationFlowContext integrationFlowContext;

    @Test
    public void testEnrichSecondary() {
        when(restTemplate.exchange(..... arg matchers ....)).thenReturn(
              new ResponseEntity("test document", HttpStatus.OK)
        );

        final Consumer<EnricherSpec> enrichSecondary = xxIntegrationConfig.enrichSecondary();
        IntegrationFlow flow =
            f -> f.enrich(enrichSecondary());
        IntegrationFlowContext.IntegrationFlowRegistration flowRegistration =
            integrationFlowContext.registration(flow).register();

        final Message<?> request =
                MessageBuilder.withPayload(new HashMap<String,Object>())
                        .setHeader("mainId", "xx-001")
                        .build();

        Message<?> response = 
                flowRegistration.getMessagingTemplate().sendAndReceive(request); 
    }
}

This testing does not seem to override the injected RestTemplate on the config class before the beans are constructed in XXIntegrationConfig.

Any thoughts on this would be greatly appreciated.