Support @Nested tests in MockServerContainerContextCustomizerFactory

Prior to this commit, MockServerContainerContextCustomizerFactory did
not find @WebAppConfiguration on an enclosing class and therefore
failed to create a MockServerContainerContextCustomizer for a @Nested
test class.

This commit addresses this by using TestContextAnnotationUtils to
determine if the test class is "annotated" with @WebAppConfiguration.

Closes gh-29037

spring-test/src/test/java/org/springframework/test/context/web/socket/WebSocketServletServerContainerFactoryBeanTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -18,6 +18,7 @@ package org.springframework.test.context.web.socket;
 
 import javax.websocket.server.ServerContainer;
 
+import org.junit.jupiter.api.Nested;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.beans.factory.annotation.Autowired;
@@ -46,6 +47,20 @@ class WebSocketServletServerContainerFactoryBeanTests {
 		assertThat(serverContainer.getDefaultMaxTextMessageBufferSize()).isEqualTo(42);
 	}
 
+	/*
+	 * @Nested test class to verify that the MockServerContainerContextCustomizerFactory
+	 * properly supports finding @WebAppConfiguration on an enclosing class.
+	 */
+	@Nested
+	class NestedTests {
+
+		@Test  // gh-29037
+		void servletServerContainerFactoryBeanSupport(@Autowired ServerContainer serverContainer) {
+			assertThat(serverContainer.getDefaultMaxTextMessageBufferSize()).isEqualTo(42);
+		}
+
+	}
+
 
 	@Configuration
 	@EnableWebSocket