Change override_settings to do its subclass check lazily

[Chris Jerdonek]

Currently, the @override_settings decorator raises a ValueError if it's used to decorate a class that doesn't inherit from SimpleTestCase:
​https://github.com/django/django/blob/56f9579105c324ff15250423bf9f8bdf1634cfb4/django/test/utils.py#L519-L521

However, this prevents the decorator from being useable as a test-class mixin for a number of SimpleTestCase subclasses. When creating shared test-class functionality, it's often better to use a mixin rather than a concrete TestCase class. This way the mixin won't have its setUp methods run unnecessarily on a test-case class with no tests, and it won't count towards the parallel test runner's test-case class count, etc.

The check could instead be done lazily, e.g. inside setUpClass(), while still having the same desired effect.

I noticed this when seeing if I could convert AuthViewsTestCase into a mixin:
​https://github.com/django/django/blob/56f9579105c324ff15250423bf9f8bdf1634cfb4/tests/auth_tests/test_views.py
The AuthViewsTestCase change could be done in a nicer way once this ticket is implemented, perhaps even as part of this ticket.