Fix addFailTest

The addFailTest method needed to be updated to catch up with some
test infrastructure changes.  Rather than consulting the merger's
git repo for the ZUUL_REF to find out if a job which is supposed to
fail is present for a given build, use the new FakeBuild.hasChanges
method to inspect the launcher's prepared git repo.

Also rename the method as 'Job' is more descriptive than 'Test' and
document it.

Change-Id: I3224b8a01d49cfa06b799a8028c1bf0d455d25b1
diff --git a/tests/test_connection.py b/tests/test_connection.py
index 6d1514b..c128741 100644
--- a/tests/test_connection.py
+++ b/tests/test_connection.py
@@ -53,7 +53,7 @@
                          'jenkins')
 
         B = self.fake_review_gerrit.addFakeChange('org/project', 'master', 'B')
-        self.worker.addFailTest('project-test2', B)
+        self.launch_server.failJob('project-test2', B)
         self.addEvent('review_gerrit', B.getPatchsetCreatedEvent(1))
 
         self.waitUntilSettled()