[1.11.x] Refs #27790 -- Reverted "Removed available_apps on TestCase subclasses."

This reverts commit 91023d79ec as it
increases memory usage for the test suite.

Backport of 7d50d2bcb8 from master

tests/multiple_database/tests.py

@@ -1794,6 +1794,12 @@ class SyncOnlyDefaultDatabaseRouter(object):
 
 class MigrateTestCase(TestCase):
 
+    # Limit memory usage when calling 'migrate'.
+    available_apps = [
+        'multiple_database',
+        'django.contrib.auth',
+        'django.contrib.contenttypes'
+    ]
     multi_db = True
 
     def test_migrate_to_other_database(self):

tests/swappable_models/tests.py

@@ -11,6 +11,13 @@ from django.utils.six import StringIO
 
 class SwappableModelTests(TestCase):
 
+    # Limit memory usage when calling 'migrate'.
+    available_apps = [
+        'swappable_models',
+        'django.contrib.auth',
+        'django.contrib.contenttypes',
+    ]
+
     @override_settings(TEST_ARTICLE_MODEL='swappable_models.AlternateArticle')
     def test_generated_data(self):
         "Permissions and content types are not created for a swapped model"