[1.9.x] Refs #24919 -- Used the documented way of disabling migrations per app.

Backport of 477274acb4 from master

tests/runtests.py

@@ -153,11 +153,11 @@ def setup(verbosity, test_labels, parallel):
     settings.SITE_ID = 1
     settings.MIDDLEWARE_CLASSES = ALWAYS_MIDDLEWARE_CLASSES
     settings.MIGRATION_MODULES = {
-        # these 'tests.migrations' modules don't actually exist, but this lets
-        # us skip creating migrations for the test models.
-        'auth': 'django.contrib.auth.tests.migrations',
-        'contenttypes': 'contenttypes_tests.migrations',
-        'sessions': 'sessions_tests.migrations',
+        # This lets us skip creating migrations for the test models as many of
+        # them depend on one of the following contrib applications.
+        'auth': None,
+        'contenttypes': None,
+        'sessions': None,
     }
     log_config = copy.deepcopy(DEFAULT_LOGGING)
     # Filter out non-error logging so we don't have to capture it in lots of