Fix check for missing subheadings

If a spec heading is missing entirely, and subheadings are expected,
the heading was being used in a key lookup causing an exception.

Change-Id: I44fb2b9c701cf359433211e876d2f6b3d1dbec5c

tests/test_titles.py

@@ -64,6 +64,10 @@ class TestTitles(testtools.TestCase):
             msgs.append("Extra sections: %s" % extra_sections)
 
         for section in expect.keys():
+            # Sections missing entirely are already covered above
+            if section not in actual:
+                continue
+
             missing_subsections = [x for x in expect[section]
                                    if x not in actual[section]]
             # extra subsections are allowed