[JENKINS-20662] Add test.

test/src/test/groovy/jenkins/model/lazy/LazyBuildMixIn_Test.groovy

+package jenkins.model.lazy
+
+import hudson.model.Run
+import hudson.model.TaskListener
+import hudson.model.listeners.RunListener
+import org.junit.Rule
+import org.junit.Test
+import org.jvnet.hudson.test.Bug
+import org.jvnet.hudson.test.JenkinsRule
+
+class LazyBuildMixIn_Test {
+    @Rule
+    public JenkinsRule r = new JenkinsRule();
+
+    @Test
+    @Bug(20662)
+    public void testNewRunningBuildRelationFromPrevious() {
+        RunListener<Run> listener = new RunListener<Run>() {
+            @Override public  void onCompleted(Run r, TaskListener listener) {
+                try {
+                    Thread.sleep(1000);
+                } catch (InterruptedException e) {
+                }
+            }
+        };
+        RunListener.all().add(listener);
+        def p = r.createFreeStyleProject();
+        def b1 = p.scheduleBuild2(0).get();
+        assert null == b1.getNextBuild();
+        def b2 = p.scheduleBuild2(0).waitForStart();
+        assert b2 == b1.getNextBuild();
+    }
+}