Testing with concurrency don't clone database

[iurisilvio]

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Django setup_databases has a parallel arg to avoid running migrations for each parallel runner.

Migrations are really slow and it was critical to our migration to pytest-django. I had to run it with --nomigrations, which is not what I wanted.

[iurisilvio]

After all, I created a project to handle this issue and more. https://github.com/businho/django-migrations-ci

I think it is possible to do the parallel part on pytest-django, but I didn't try to do it.