Include setUpTestData as a way to speed up tests

[Tim Schilling]

In Adam Johnson's book "Speed up your Django Tests", there's a section on using setUpTestData to speed up tests. Since we have a section on ​speeding up tests already, I think we should add this tip to it. We also mention in docs/topics/testing/tools.txt, that setUpTestData is faster than using setUp.

[Jacob Walls]

Sounds good. I also think the AnimalTestCase example at the top of that page should be updated. It's a little debatable, since part of the point of that section is to demonstrate that TestCase is a lightweight subclass of unittest.TestCase, and we'd be immediately showing a custom method, but we should be able to clarify that with some appropriate wording.

[Jacob Walls <jacobtylerwalls@…>]

In d80ef32:

Fixed #37128 -- Doc'd setUpTestData as test speed up.

Revises the AnimalTestCase to better show test data isolation
and avoid the opportunity to use setUpTestData.

[Jacob Walls <jacobtylerwalls@…>]

In 8cfba50:

[6.1.x] Fixed #37128 -- Doc'd setUpTestData as test speed up.

Revises the AnimalTestCase to better show test data isolation
and avoid the opportunity to use setUpTestData.

Backport of d80ef32eb9e669986ed7838296f6373af56c68f0 from main.