Make TestRestTemplate not extend RestTemplate

Update TestRestTemplate so that it no longer directly extends
RestTemplate. Prior to this commit it was possible that TestRestTemplate
could interfere with user defined RestTemplate beans.

TestRestTemplate offers the same methods as RestTemplate so should be
a drop-in replacement. If access is needed to the actual underlying
template the `getRestTemplate()` method should be used.

Fixes gh-5915

spring-boot-samples/spring-boot-sample-tomcat/src/test/java/sample/tomcat/SampleTomcatApplicationTests.java

@@ -35,7 +35,6 @@ import org.springframework.http.ResponseEntity;
 import org.springframework.test.annotation.DirtiesContext;
 import org.springframework.test.context.junit4.SpringRunner;
 import org.springframework.util.StreamUtils;
-import org.springframework.web.client.RestTemplate;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -66,7 +65,7 @@ public class SampleTomcatApplicationTests {
 		HttpHeaders requestHeaders = new HttpHeaders();
 		requestHeaders.set("Accept-Encoding", "gzip");
 		HttpEntity<?> requestEntity = new HttpEntity<Object>(requestHeaders);
-		RestTemplate restTemplate = new TestRestTemplate();
+		TestRestTemplate restTemplate = new TestRestTemplate();
 		ResponseEntity<byte[]> entity = restTemplate.exchange(
 				"http://localhost:" + this.port, HttpMethod.GET, requestEntity,
 				byte[].class);