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-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/OutputCapture.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2013 the original author or authors.
+ * Copyright 2012-2016 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -26,7 +26,7 @@ import org.junit.runner.Description;
 import org.junit.runners.model.Statement;
 
 /**
- * Capture output from System.out and System.err.
+ * Internal JUnit {@code @Rule} to capture output from System.out and System.err.
  *
  * @author Phillip Webb
  */