Fixed #25290 -- Warned against modifying objects created in setUpTestData() in tests.

    (for instance, MySQL with the MyISAM engine), ``setUpTestData()`` will be

    called before each test, negating the speed benefits.

    Be careful not to modify any objects created in ``setUpTestData()`` in

    your test methods. Modifications to in-memory objects from setup work done

    at the class level will persist between test methods. If you do need to

    modify them, you could reload them in the ``setUp()`` method with

    :meth:`~django.db.models.Model.refresh_from_db`, for example.

.. warning::

    If you want to test some specific database transaction behavior, you should