Make TestCase.setUpTestData enforce in-memory data isolation.

[Simon Charette]

​The following is copied from a two years old django-developers thread that received an initial warm welcome.

Since then the ​third-party app implementing this feature has not received much feedback but I know a few developers have been using it and my usage in a few projects didn't surface any issues.

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Django 1.8 introduced the TestCase.setUpTestData() class method as a mean to
speed up test fixtures initialization as compared to using setUp().

As I've come to use this feature and review changes from peers using it in
different projects the fact that test data assigned during its execution
couldn't be safely altered by test methods without compromising test isolation
has often be the source of confusion and frustration.

While the setUpTestData documentation mentions this limitation[1] and ways to
work around it by using refresh_from_db() in setUp() I believe it defeats
the whole purpose of the feature; avoiding unnecessary roundtrips to the
database to speed up execution. Given TestCase goes through great lengths to
ensure database level data isolation I believe it should do the same with class
level in-memory data assigned during setUpTestData.

In order to get rid of this caveat of the feature I'd like to propose an
adjustment to ensure such in-memory test data isolation.

[Mariusz Felisiak]

​PR

[Mariusz Felisiak <felisiak.mariusz@…>]

In 3cf80d3f:

Fixed #31395 -- Made setUpTestData enforce in-memory data isolation.

Since it's introduction in Django 1.8 setUpTestData has been suffering
from a documented but confusing caveat due to its sharing of attributes
assigned during its execution with all test instances.

By keeping track of class attributes assigned during the setUpTestData
phase its possible to ensure only deep copies are provided to test
instances on attribute retreival and prevent manual setUp gymnastic to
work around the previous lack of in-memory data isolation.

Thanks Adam Johnson for the extensive review.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 94f63b92:

Refs #31395 -- Relied on setUpTestData() test data isolation in various tests.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 2e10abe:

Refs #31395 -- Removed support for assigning objects which don't support deepcopy() in setUpTestData().

Per deprecation timeline.