Add a new spring-boot-sample-test application

Add a sample application that demonstrates recently added testing
features.

See gh-4901

spring-boot-samples/pom.xml

@@ -76,6 +76,7 @@
 		<module>spring-boot-sample-servlet</module>
 		<module>spring-boot-sample-session-redis</module>
 		<module>spring-boot-sample-simple</module>
+		<module>spring-boot-sample-test</module>
 		<module>spring-boot-sample-testng</module>
 		<module>spring-boot-sample-tomcat</module>
 		<module>spring-boot-sample-tomcat-jsp</module>