Configure TestRestTemplate using builder

Update SpringBootTestContextCustomizer to create the TestRestTemplate
using the RestTemplateBuilder whenever possible.

Fixes gh-5509

spring-boot-test/src/test/java/org/springframework/boot/test/context/AbstractSpringBootTestEmbeddedWebEnvironmentTests.java

@@ -60,6 +60,10 @@ public abstract class AbstractSpringBootTestEmbeddedWebEnvironmentTests {
 	@Autowired
 	private TestRestTemplate restTemplate;
 
+	public TestRestTemplate getRestTemplate() {
+		return this.restTemplate;
+	}
+
 	@Test
 	public void runAndTestHttpEndpoint() {
 		assertThat(this.port).isNotEqualTo(8080).isNotEqualTo(0);

spring-boot-test/src/test/java/org/springframework/boot/test/context/SpringBootTestWebEnvironmentRandomPortTests.java

@@ -16,15 +16,21 @@
 
 package org.springframework.boot.test.context;
 
+import org.junit.Test;
 import org.junit.runner.RunWith;
 
 import org.springframework.boot.test.context.SpringBootTest.WebEnvironment;
+import org.springframework.boot.web.client.RestTemplateBuilder;
+import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.http.converter.StringHttpMessageConverter;
 import org.springframework.test.annotation.DirtiesContext;
 import org.springframework.test.context.junit4.SpringRunner;
 import org.springframework.web.bind.annotation.RestController;
 import org.springframework.web.servlet.config.annotation.EnableWebMvc;
 
+import static org.assertj.core.api.Assertions.assertThat;
+
 /**
  * Tests for {@link SpringBootTest} configured with {@link WebEnvironment#RANDOM_PORT}.
  *
@@ -37,11 +43,28 @@ import org.springframework.web.servlet.config.annotation.EnableWebMvc;
 public class SpringBootTestWebEnvironmentRandomPortTests
 		extends AbstractSpringBootTestEmbeddedWebEnvironmentTests {
 
+	@Test
+	public void testRestTemplateShouldUseBuilder() throws Exception {
+		assertThat(getRestTemplate().getRestTemplate().getMessageConverters())
+				.hasAtLeastOneElementOfType(MyConverter.class);
+	}
+
 	@Configuration
 	@EnableWebMvc
 	@RestController
 	protected static class Config extends AbstractConfig {
 
+		@Bean
+		public RestTemplateBuilder restTemplateBuilder() {
+			return new RestTemplateBuilder()
+					.additionalMessageConverters(new MyConverter());
+
+		}
+
+	}
+
+	private static class MyConverter extends StringHttpMessageConverter {
+
 	}
 
 }