GH-2112 fix test configuration

spring-cloud-stream/src/test/java/org/springframework/cloud/stream/function/ScenarioTests.java

@@ -29,6 +29,7 @@ import org.springframework.cloud.stream.binder.test.TestChannelBinderConfigurati
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.messaging.Message;
 import org.springframework.messaging.support.GenericMessage;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -80,6 +81,15 @@ public class ScenarioTests {
 		}
 	}
 
+	@EnableAutoConfiguration
+	@Configuration
+	public static class TestConfiguration {
+		@Bean
+		public Function<Message<String>, Message<String>> messageFunction() {
+			return v -> v;
+		}
+	}
+
 	@EnableAutoConfiguration
 	@Configuration
 	public static class FunctionReturningNullConfiguration {