[1.8.x] Cleaned up migration writer tests
[1.8.x] Cleaned up migration writer tests

Backport of 65d55c409343aab7c2ae771c459720ef797b4cdb from master

h4r5h1t/django-hauthy,h4r5h1t/django-hauthy,pquentin/django,h4r5h1t/django-hauthy,pquentin/django,h4r5h1t/django-hauthy,pquentin/django