[JENKINS-18074] Fixed unit test by making StubJob.getParent forgiving.

https://ci.jenkins-ci.org/job/jenkins_main_trunk/2545/testReport/junit/hudson.model/DisplayNameListenerTest/testOnCopied/

core/src/test/java/hudson/model/StubJob.java

@@ -28,8 +28,9 @@ import java.util.SortedMap;
 
 /**
  * @author kingfai
- *
+ * @deprecated Does not behave very consistently. Either write a real functional test with {@code JenkinsRule}, or use PowerMock/Mockito.
  */
+@Deprecated
 @SuppressWarnings({ "rawtypes", "unchecked" })
 class StubJob extends Job {
 
@@ -69,4 +70,8 @@ class StubJob extends Job {
         
     }   
 
+    @Override public ItemGroup getParent() {
+        return null;
+    }
+
 }