Django 5.2 changes _pre_setup to a classmethod

In Django, as of https://github.com/django/django/commit/8eca3e9bce519c21340312ee7846c92b27abea79 , the `_pre_setup` method of the SimpleTestCase has changed from an instance method to a class method. The definition of `_pre_setup` in the `MigratorTestCase` class in the `contrib/unittest_case.py` file needs to change accordingly.