Fixed #23727 -- Inhibited the post_migrate signal when using serialized_rollback.

When using a TransactionTestCase with serialized_rollback=True,
after creating the database and running its migrations (along with
emitting the post_migrate signal), the contents of the database
are serialized to _test_serialized_contents.

After the first test case, _fixture_teardown() would flush the
tables but then the post_migrate signal would be emitted and new
rows (with new PKs) would be created in the django_content_type
table. Then in any subsequent test cases in a suite,
_fixture_setup() attempts to deserialize the content of
 _test_serialized_contents, but these rows are identical to the
rows already in the database except for their PKs.  This causes an
IntegrityError due to the unique constraint in the
django_content_type table.

This change made it so that in the above scenario the post_migrate
signal is not emitted after flushing the tables, since it will be
repopulated during fixture_setup().

tests/test_utils/test_transactiontestcase.py

@@ -0,0 +1,28 @@
+from django.test import TransactionTestCase, mock
+
+
+class TestSerializedRollbackInhibitsPostMigrate(TransactionTestCase):
+    """
+    TransactionTestCase._fixture_teardown() inhibits the post_migrate signal
+    for test classes with serialized_rollback=True.
+    """
+    available_apps = ['test_utils']
+    serialized_rollback = True
+
+    def setUp(self):
+        # self.available_apps must be None to test the serialized_rollback
+        # condition.
+        self.available_apps = None
+
+    def tearDown(self):
+        self.available_apps = ['test_utils']
+
+    @mock.patch('django.test.testcases.call_command')
+    def test(self, call_command):
+        # with a mocked call_command(), this doesn't have any effect.
+        self._fixture_teardown()
+        call_command.assert_called_with(
+            'flush', interactive=False, allow_cascade=False,
+            reset_sequences=False, inhibit_post_migrate=True,
+            database='default', verbosity=0,
+        )