Automatically run checks at the start of tests

I've been trying to get Django's test runner to run manage.py check at the start of the test suite in django/django#6294. It actually used to do it in 1.7 and then no one noticed it disappearing in 1.8. There is already a solution in that PR but it requires a lot of secondary effort to fix all the check failures that appear in Django's own test suite.

The reason to run check is to avoid wasting time on tests when there is a setup related issue that shoudl be fixed first. Since tests don't run checks by default, TDD-only workflows (i.e. no runserver etc.) can miss check errors until e.g. CI runs them explicitly, which can waste effort. Also they take a trivial amount of time to run so don't slow tests down really.

I think it would probably be easier to add it to pytest-django before I finish my Django core PR. Also we're using pytest-django at work now, so it would fix it for me for the time being.

Start by locating pytest-django's test-runner integration and the entry point that invokes Django's test suite. Compare its behavior with manage.py check and review the existing solution in django/django#6294. Done means checks run at the start of the suite without requiring separate setup, with the relevant plugin tests passing.