Remove WebClient's in-memory buffer size limit for endpoint tests

Previously, the thread dump endpoint's response could exceed
WebClient's in-memory buffer limt when there were a large number of
threads or the threads had large stacks.

This commit disables WebClient's in-memory buffer size limit so that
the test passing is not dependent on the number of active threads and
their stack sizes.

Closes gh-22101

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/endpoint/web/test/WebEndpointTestInvocationContextProvider.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.
@@ -192,7 +192,8 @@ class WebEndpointTestInvocationContextProvider implements TestTemplateInvocation
 			DefaultUriBuilderFactory uriBuilderFactory = new DefaultUriBuilderFactory(
 					"http://localhost:" + determinePort());
 			uriBuilderFactory.setEncodingMode(EncodingMode.NONE);
-			return WebTestClient.bindToServer().uriBuilderFactory(uriBuilderFactory).responseTimeout(TIMEOUT).build();
+			return WebTestClient.bindToServer().uriBuilderFactory(uriBuilderFactory).responseTimeout(TIMEOUT)
+					.codecs((codecs) -> codecs.defaultCodecs().maxInMemorySize(-1)).build();
 		}
 
 		private int determinePort() {