Refs #27790 -- Reverted "Removed available_apps on TestCase subclasses."

This reverts commit 91023d79ec70df9289271e63a67675ee51e7dea8 as it
increases memory usage for the test suite.

tests/multiple_database/tests.py

@@ -1792,6 +1792,12 @@ class SyncOnlyDefaultDatabaseRouter:
 
 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

@@ -10,6 +10,13 @@ from .models import Article
 
 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"