printout testr tests that fail

We test whether or not testr fails to list all the tests, however
we're not being very nice about reporting when it fails (especially
not what caused the failure), and throw a cryptic 0 != 3 mismatch
error.

This should help make it easy to find syntax issues in the future.

Change-Id: I0d4b5b2a280813d54b0c2712a504e472370bd196

tempest/tests/test_list_tests.py

@@ -31,7 +31,7 @@ class TestTestList(base.TestCase):
         ids, err = p.communicate()
         self.assertEqual(0, p.returncode,
                          "test discovery failed, one or more files cause an "
-                         "error on import")
+                         "error on import %s" % ids)
         ids = ids.split('\n')
         for test_id in ids:
             if re.match('(\w+\.){3}\w+', test_id):