🔙 Enable serialized_rollback in TestCases

[coiax]

https://docs.djangoproject.com/en/2.2/topics/testing/overview/#rollback-emulation

When using the transactional_db or live_server fixtures, during cleanup, Django will truncate the tables.
If you have data migrations that provide initial data, that initial data will be wiped as well.

By using the serialized_rollback attribute in TestCases, it should enable you to load the initial data at the start of every test, but using pytest-django means you can't edit the TestCase classes directly.

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I propose a mark, django_serialized_rollback (?), which if present, will enable the serialized_rollback attribute, thus enabling this behavior.

Wanted to open an issue first to check that was the right way of going about this, before actually writing the tests and code for it.

[luzfcb]

@coiax feel free to continue the work of #721

[coiax]

Closing, since this is a duplicate of #329