Removed superfluous models.py files.

Added comments in the three empty models.py files that are still needed.

Adjusted the test runner to add applications corresponding to test
labels to INSTALLED_APPS even when they don't have a models module.

django/contrib/admindocs/models.py

-# Empty models.py to allow for specifying admindocs as a test label.

django/contrib/formtools/models.py

-""" models.py (even empty) currently required by the runtests.py to enable unit tests """
+# This file is required to pretend formtools has models.
+# Otherwise test models cannot be registered.

django/contrib/messages/models.py

-# Models module required so tests are discovered.

django/contrib/sitemaps/models.py

-# This file intentionally left blank
+# This file is required to pretend sitemaps has models.
+# Otherwise test models cannot be registered.

tests/admin_scripts/custom_templates/app_template/models.py

-# whatever

tests/context_processors/models.py

-# Models file for tests to run.

tests/csrf_tests/models.py

-# models.py file for tests to run.

tests/decorators/models.py

-# A models.py so that tests run.

tests/i18n/resolution/models.py

-#

tests/migrate_signals/models.py

+# Remove this module when pre/post_migrate are refactored to use something
+# other than a models module for their "sender" argument.

tests/requests/models.py

-# Need a models module for the test runner.

tests/runtests.py

@@ -164,8 +164,7 @@ def setup(verbosity, test_labels):
         if module_found_in_labels:
             if verbosity >= 2:
                 print("Importing application %s" % module_name)
-            mod = app_cache.load_app(module_label)
-            if mod:
+            app_cache.load_app(module_label)
             if module_label not in settings.INSTALLED_APPS:
                 settings.INSTALLED_APPS.append(module_label)
 

tests/signed_cookies_tests/models.py

-# models.py file for tests to run.

tests/signing/models.py

-# models.py file for tests to run.