Make it easier to add auto-configuration to a test slice

Previously, an entry had to be added to spring.factories using the
name of one of the @AutoConfigure… meta-annotations on the @…Test
annotation as the key. This indirection was unnecessarily complicated.

This commit simplifies things by allowing the name of the @…Test
annotation itself to be used as the key.

Closes gh-6335

spring-boot-samples/spring-boot-sample-devtools/src/main/java/sample/devtools/MyController.java

@@ -18,6 +18,7 @@ package sample.devtools;
 
 import java.util.Date;
 
+import javax.annotation.PostConstruct;
 import javax.servlet.http.HttpSession;
 
 import org.springframework.stereotype.Controller;
@@ -28,6 +29,11 @@ import org.springframework.web.servlet.ModelAndView;
 @Controller
 public class MyController {
 
+	@PostConstruct
+	public void slowRestart() throws InterruptedException {
+		Thread.sleep(5000);
+	}
+
 	@GetMapping("/")
 	public ModelAndView get(HttpSession session) {
 		Object sessionVar = session.getAttribute("var");