Check that git clone against x/ project succeeds in testing

Previously the test was checking that stderr reported "Cloning into
$PATH" which also happens in failure cases. We add an explicit check for
a successful command return code to ensure that we aren't failing with
that output.

Change-Id: Iec51217f2cc97e6a56ff9d8b7a260650010f229f

testinfra/test_gerrit.py

@@ -30,6 +30,7 @@ def test_gerrit_x_project_clone(host):
     cmd = host.run(
         'git clone http://localhost:8081/x/test-project /tmp/test-project')
     assert "Cloning into '/tmp/test-project'..." in cmd.stderr
+    assert cmd.succeeded
 
 def test_gerrit_screenshot(host):
     driver = webdriver.Remote(