Restructure spring-boot-test packages

Create a new package structure for `spring-boot-test` and deprecate
existing classes.

Fixes gh-5293

spring-boot-test/pom.xml

@@ -30,6 +30,11 @@
 			<artifactId>javax.servlet-api</artifactId>
 			<optional>true</optional>
 		</dependency>
+		<dependency>
+			<groupId>junit</groupId>
+			<artifactId>junit</artifactId>
+			<optional>true</optional>
+		</dependency>
 		<dependency>
 			<groupId>org.apache.httpcomponents</groupId>
 			<artifactId>httpclient</artifactId>