Support WebSocket ServletServerContainerFB in the TCF

Prior to this commit, any attempt to include a bean of type
ServletServerContainerFactoryBean in the WebApplicationContext for an
integration test class annotated with @WebAppConfiguration in
conjunction the Spring TestContext Framework (TCF) would have resulted
in an IllegalStateException stating that "A ServletContext is required
to access the javax.websocket.server.ServerContainer instance."

In such scenarios, the MockServletContext was in fact present in the
WebApplicationContext; however there was no WebSocket ServerContainer
stored in the ServletContext.

This commit addresses this issue by introducing the following.

- MockServerContainer: a private mock implementation of the
  javax.websocket.server.ServerContainer interface.

- MockServerContainerContextCustomizer: a ContextCustomizer that
  instantiates a new MockServerContainer and stores it in the
  ServletContext under the attribute named
  "javax.websocket.server.ServerContainer".

- MockServerContainerContextCustomizerFactory: a
  ContextCustomizerFactory which creates a
  MockServerContainerContextCustomizer if WebSocket support is present
  in the classpath and the test class is annotated with
  @WebAppConfiguration. This factory is registered by default via the
  spring.factories mechanism.

Issue: SPR-14367

spring-websocket/src/main/java/org/springframework/web/socket/server/standard/ServletServerContainerFactoryBean.java

@@ -53,7 +53,7 @@ public class ServletServerContainerFactoryBean
 	private Integer maxTextMessageBufferSize;
 
 	private Integer maxBinaryMessageBufferSize;
-	
+
 	private ServletContext servletContext;
 
 	private ServerContainer serverContainer;