[JENKINS-44657] Move sub-job's buildsdir when outside of itemrootdir (#2932)

* A test to verify JENKINS-44657 is working in core

* [JENKINS-44657] Move sub-job's buildsdir when outside of itemrootdir.

* [JENKINS-44657] Fix UsageStatisticsTest

Because we've added a test dependency that affects it.

* [JENKINS-44657] Normalize the path before comparing

* [JENKINS-44657] Use MockFolder instead of folders-plugin

for tests

* [JENKINS-44657] Document and annotate isDefaultBuildDir

* [JENKINS-44657] isDescendant doesn't throw IOException any more

* [JENKINS-44657] catch InvalidPathException and rethrow as IOException

core/src/main/java/hudson/Util.java

@@ -567,6 +567,25 @@ public class Util {
         return true;
     }
 
+    /**
+     * A check if a file path is a descendant of a parent path
+     * @param forParent the parent the child should be a descendant of
+     * @param potentialChild the path to check
+     * @return true if so
+     * @throws IOException for invalid paths
+     * @since FIXME
+     * @see InvalidPathException
+     */
+    public static boolean isDescendant(File forParent, File potentialChild) throws IOException {
+        try {
+            Path child = potentialChild.getAbsoluteFile().toPath().normalize();
+            Path parent = forParent.getAbsoluteFile().toPath().normalize();
+            return child.startsWith(parent);
+        } catch (InvalidPathException e) {
+            throw new IOException(e);
+        }
+    }
+
     /**
      * Creates a new temporary directory.
      */