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-full-build/pom.xml

@@ -49,6 +49,7 @@
 		<module>../spring-boot-parent</module>
 		<module>../spring-boot-tools</module>
 		<module>../spring-boot</module>
+		<module>../spring-boot-test</module>
 		<module>../spring-boot-autoconfigure</module>
 		<module>../spring-boot-actuator</module>
 		<module>../spring-boot-actuator-docs</module>