Extract spring-boot-test.jar

Relocate the `org.springframework.boot.test` package from the
`spring-boot.jar` to `spring-boot-test.jar`.

Fixes gh-5184

spring-boot-starters/spring-boot-starter-test/pom.xml

@@ -18,6 +18,10 @@
 		<main.basedir>${basedir}/../..</main.basedir>
 	</properties>
 	<dependencies>
+		<dependency>
+			<groupId>org.springframework.boot</groupId>
+			<artifactId>spring-boot-test</artifactId>
+		</dependency>
 		<dependency>
 			<groupId>junit</groupId>
 			<artifactId>junit</artifactId>