Refactor WebSocket int. tests to work w/ Jetty 9.3

Recent builds of Jetty 9.3 require that Jetty's own ServletContext
implementation be supplied to WebSocketServerFactory's init() method.
Otherwise, the Jetty server will fail to start with the exception
message: "Not running on Jetty, WebSocket support unavailable".

This commit refactors AbstractWebSocketIntegrationTests,
AbstractSockJsIntegrationTests, and all WebSocketTestServer
implementations in order to support this new requirement.

Specifically:

- WebSocketTestServer defines a new getServletContext() method;
  TomcatWebSocketTestServer, UndertowTestServer, and
  JettyWebSocketTestServer have all been updated to return the
  ServletContext created by the embedded server.

- The setup() methods in AbstractWebSocketIntegrationTests and
  AbstractSockJsIntegrationTests have been updated so that the
  WebApplicationContext is supplied the appropriate ServletContext,
  after deployConfig() has been invoked on the WebSocketTestServer but
  before the WebApplicationContext is refreshed.

Issue: SPR-13162

spring-websocket/src/test/resources/log4j.properties

@@ -1,9 +1,8 @@
 log4j.appender.console=org.apache.log4j.ConsoleAppender
 log4j.appender.console.layout=org.apache.log4j.PatternLayout
-log4j.appender.console.layout.ConversionPattern=%d{HH:mm:ss,SSS} [%c][%t] - %m%n
+log4j.appender.console.layout.ConversionPattern=%d{HH:mm:ss,SSS} [%-5p] [%c] - %m%n
 
 log4j.rootCategory=WARN, console
-log4j.logger.org.springframework.web=DEBUG
-log4j.logger.org.springframework.web.socket=TRACE
-log4j.logger.org.springframework.messaging=DEBUG
-
+log4j.logger.org.springframework.web=WARN
+log4j.logger.org.springframework.web.socket=WARN
+log4j.logger.org.springframework.messaging=WARN