Test and update fixture loading, in MySQL and other engines

[jwhitlock]

_foreign_key_ignoring_handle in django_nose.runner has this comment:

django-nose/django_nose/runner.py

Lines 316 to 321 in 5e824f5

	def _foreign_key_ignoring_handle(self, *fixture_labels, **options):
	"""Wrap the the stock loaddata to ignore foreign key checks.
	This allows loading circular references from fixtures, and is
	monkeypatched into place in setup_databases().
	"""

This was brought over in #50, a large refactor to merge in the test-utils project.

This code presented problems when fixtures were loaded with Django 2.0, which were tracked down to assumptions about the commit argument, removed in Django 1.5 (see #307 (comment)). The code was fixed in #308, and @dlareau suggested the code should be revisited (see #307 (comment)).

It is possible that modern Django has other methods for loading fixtures while avoiding circular imports, and _foreign_key_ignoring_handle should be retired. A good first step would be to add a django-nose test that exercises loading fixtures, ideally with a circular reference.