Make assertable context interfaces configurable

Update `Assertable*ApplicationContext` interfaces so that they also
extend the appropriate `Configurable*ApplicationContext` interface.

Closes gh-14650

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/assertj/AssertableApplicationContext.java

 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -35,7 +35,8 @@ import org.springframework.context.ConfigurableApplicationContext;
  * @see ApplicationContext
  */
 public interface AssertableApplicationContext
-		extends ApplicationContextAssertProvider<ConfigurableApplicationContext> {
+		extends ApplicationContextAssertProvider<ConfigurableApplicationContext>,
+		ConfigurableApplicationContext {
 
 	/**
 	 * Factory method to create a new {@link AssertableApplicationContext} instance.

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/assertj/AssertableReactiveWebApplicationContext.java

 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -35,7 +35,7 @@ import org.springframework.boot.web.reactive.context.ReactiveWebApplicationConte
  */
 public interface AssertableReactiveWebApplicationContext extends
 		ApplicationContextAssertProvider<ConfigurableReactiveWebApplicationContext>,
-		ReactiveWebApplicationContext {
+		ConfigurableReactiveWebApplicationContext {
 
 	/**
 	 * Factory method to create a new {@link AssertableReactiveWebApplicationContext}

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/assertj/AssertableWebApplicationContext.java

 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -36,7 +36,7 @@ import org.springframework.web.context.WebApplicationContext;
  */
 public interface AssertableWebApplicationContext
 		extends ApplicationContextAssertProvider<ConfigurableWebApplicationContext>,
-		WebApplicationContext {
+		ConfigurableWebApplicationContext {
 
 	/**
 	 * Factory method to create a new {@link AssertableWebApplicationContext} instance.