Fixed #25251 -- Made data migrations available in TransactionTestCase when using --keepdb.

Data loaded in migrations were restored at the beginning of each
TransactionTestCase and all the tables are truncated at the end of
these test cases. If there was a TransactionTestCase at the end of
the test suite, the migrated data weren't restored in the database
(especially unexpected when using --keepdb). Now data is restored
at the end of each TransactionTestCase.

tests/test_utils/test_transactiontestcase.py

@@ -1,10 +1,44 @@
+import json
 from unittest import mock
 
+from django.apps import apps
 from django.db import connections
 from django.test import TestCase, TransactionTestCase, override_settings
 
+from .models import Car
 
-class TestSerializedRollbackInhibitsPostMigrate(TransactionTestCase):
+
+class TestSerializedContentMockMixin:
+    """
+    Use this mixin on each test involving TransactionTestCase and
+    serialized_rollback = True option to avoid test dependencies. It mocks what
+    would be serialized after initial data migrations and restores it at the
+    end of the test.
+    """
+    initial_data_migration = '[]'
+    _connections_test_serialized_content = {}
+
+    def _pre_setup(self):
+        for db_name in self._databases_names(include_mirrors=False):
+            self._connections_test_serialized_content[db_name] = connections[db_name]._test_serialized_contents
+            connections[db_name]._test_serialized_contents = self.initial_data_migration
+        super()._pre_setup()
+
+    def _post_teardown(self):
+        super()._post_teardown()
+        for db_name in self._databases_names(include_mirrors=False):
+            connections[db_name]._test_serialized_contents = self._connections_test_serialized_content[db_name]
+
+    @classmethod
+    def tearDownClass(cls):
+        super().tearDownClass()
+        # Clean up any data that has been created by the class.
+        for data in json.loads(cls.initial_data_migration):
+            model = apps.get_model(*data['model'].split('.'))
+            model.objects.filter(pk=data['pk']).delete()
+
+
+class TestSerializedRollbackInhibitsPostMigrate(TestSerializedContentMockMixin, TransactionTestCase):
     """
     TransactionTestCase._fixture_teardown() inhibits the post_migrate signal
     for test classes with serialized_rollback=True.
@@ -44,3 +78,39 @@ class TransactionTestCaseMultiDbTests(TestCase):
         """
         for alias in connections:
             self.assertEqual(len(connections[alias].queries_log), 0, 'Failed for alias %s' % alias)
+
+
+class TestDataRestoredOnTearDownIfSerializedRollback(TestSerializedContentMockMixin, TransactionTestCase):
+    """
+    Initial data is recreated in TransactionTestCase._fixture_teardown()
+    after the database is flushed so it's available in next test.
+    """
+    available_apps = ['test_utils']
+    _next_serialized_rollback = True
+    initial_data_migration = '[{"model": "test_utils.car", "pk": 666, "fields": {"name": "K 2000"}}]'
+
+    def _post_teardown(self):
+        super()._post_teardown()
+        # Won't be True if running the tests with --reverse.
+        if self._next_serialized_rollback:
+            self.assertTrue(Car.objects.exists())
+
+    def test(self):
+        pass  # Should be the only one in this class.
+
+
+class TestDataNotRestoredOnTearDownIfNotSerializedRollback(TestSerializedContentMockMixin, TransactionTestCase):
+    """
+    Initial data isn't recreated in TransactionTestCase._fixture_teardown()
+    if _next_serialized_rollback is False.
+    """
+    available_apps = ['test_utils']
+    _next_serialized_rollback = False
+    initial_data_migration = '[{"model": "test_utils.car", "pk": 666, "fields": {"name": "K 2000"}}]'
+
+    def _post_teardown(self):
+        super()._post_teardown()
+        self.assertFalse(Car.objects.exists())
+
+    def test(self):
+        pass  # Should be the only one in this class.