Improved isolation of applied-migrations table in migration tests.

tests/migrations/test_base.py

@@ -6,6 +6,7 @@ from importlib import import_module
 
 from django.apps import apps
 from django.db import connection
+from django.db.migrations.recorder import MigrationRecorder
 from django.test import TransactionTestCase
 from django.test.utils import extend_sys_path
 from django.utils.module_loading import module_dir
@@ -18,6 +19,11 @@ class MigrationTestBase(TransactionTestCase):
 
     available_apps = ["migrations"]
 
+    def tearDown(self):
+        # Reset applied-migrations state.
+        recorder = MigrationRecorder(connection)
+        recorder.migration_qs.filter(app='migrations').delete()
+
     def get_table_description(self, table):
         with connection.cursor() as cursor:
             return connection.introspection.get_table_description(cursor, table)