demo of using currentBuild.rawBuild.parent to do lookups of sister jobs by name

test.java

    @Test public void lookUpSister() {
        r.addStep(new Statement() {
            @Override public void evaluate() throws Throwable {
                MockFolder dir = r.j.createFolder("dir");
                WorkflowJob ds = dir.createProject(WorkflowJob.class, "ds");
                ds.setDefinition(new CpsFlowDefinition("echo 'ran downstream build'"));
                r.j.assertBuildStatusSuccess(ds.scheduleBuild2(0));
                WorkflowJob us = dir.createProject(WorkflowJob.class, "us");
                us.setDefinition(new CpsFlowDefinition(
                    "def jobName = 'ds'\n" +
                    "getLogs(jobName)\n" +
                    "String getLogs (String jobName) {\n" +
                    "  def job = Jenkins.instance.getItem(jobName, currentBuild.rawBuild.parent)\n" +
                    "  echo job.getLastBuild().getLog()\n" +
                    "}"));
                r.j.assertLogContains("ran downstream build", r.j.assertBuildStatusSuccess(us.scheduleBuild2(0)));
            }
        });
    }