Make TestRestTemplate withBasicAuth fault tolerant

It's part of TestRestTemplate's contract that it's fault tolerant. This
commit aligns the behavior of `withBasicAuth` with that expectation.

Closes gh-15780

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/web/client/TestRestTemplate.java

@@ -1034,11 +1034,8 @@ public class TestRestTemplate {
 				.messageConverters(getRestTemplate().getMessageConverters())
 				.interceptors(getRestTemplate().getInterceptors())
 				.uriTemplateHandler(getRestTemplate().getUriTemplateHandler()).build();
-		TestRestTemplate testRestTemplate = new TestRestTemplate(restTemplate, username,
-				password, this.httpClientOptions);
-		testRestTemplate.getRestTemplate()
-				.setErrorHandler(getRestTemplate().getErrorHandler());
-		return testRestTemplate;
+		return new TestRestTemplate(restTemplate, username, password,
+				this.httpClientOptions);
 	}
 
 	@SuppressWarnings({ "rawtypes", "unchecked" })

spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/web/client/TestRestTemplateTests.java

@@ -237,14 +237,15 @@ public class TestRestTemplateTests {
 	}
 
 	@Test
-	public void withBasicAuthDoesNotResetErrorHandler() {
+	public void withBasicAuthShouldUseNoOpErrorHandler() throws Exception {
 		TestRestTemplate originalTemplate = new TestRestTemplate("foo", "bar");
 		ResponseErrorHandler errorHandler = mock(ResponseErrorHandler.class);
 		originalTemplate.getRestTemplate().setErrorHandler(errorHandler);
 		TestRestTemplate basicAuthTemplate = originalTemplate.withBasicAuth("user",
 				"password");
 		assertThat(basicAuthTemplate.getRestTemplate().getErrorHandler())
-				.isSameAs(errorHandler);
+				.isInstanceOf(Class.forName(
+						"org.springframework.boot.test.web.client.TestRestTemplate$NoOpResponseErrorHandler"));
 	}
 
 	@Test