Add @FunctionalInterface to remaining public and protected interfaces

See gh-6857

spring-boot-test/src/main/java/org/springframework/boot/test/context/DefaultTestExecutionListenersPostProcessor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2016 the original author or authors.
+ * Copyright 2012-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -29,6 +29,7 @@ import org.springframework.test.context.TestExecutionListener;
  * @since 1.4.1
  * @see SpringBootTest
  */
+@FunctionalInterface
 public interface DefaultTestExecutionListenersPostProcessor {
 
 	/**