[1.8.x] Improved isolation of applied-migrations table in migration tests.

Backport of 335fc44f68 from master.

tests/migrations/test_base.py

@@ -1,6 +1,7 @@
 import os
 
 from django.db import connection
+from django.db.migrations.recorder import MigrationRecorder
 from django.test import TransactionTestCase
 from django.utils._os import upath
 
@@ -13,6 +14,11 @@ class MigrationTestBase(TransactionTestCase):
     available_apps = ["migrations"]
     test_dir = os.path.abspath(os.path.dirname(upath(__file__)))
 
+    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)