Fix the checking of directories for extensions

The test for file extensions was checking everything returned by
glob.glob("guidelines/*"). The recent merge of the refactored HTTP
guidelines added a new guidelines/http/ directory, and the test was
failing because that entry didn't end with either .rst or .json. This
patch corrects that by skipping directories from that name check.

Change-Id: I0a84160015e9c1d96cb456b07c5e45fc6645df08

tests/test_titles.py

@@ -11,6 +11,7 @@
 # under the License.
 
 import glob
+import os
 import re
 
 import docutils.core
@@ -90,6 +91,8 @@ class TestTitles(testtools.TestCase):
         for file in files:
             if file.endswith('~'):
                 continue
+            if os.path.isdir(file):
+                continue
             self.assertTrue(
                 file.endswith(".rst") or file.endswith(".json"),
                 "guideline file must use 'rst' or 'json'"