It seems that migrations are unicode in Django 1.11

The idea of splitting integration from unittests is good in theory
but leads to a lot of mental overhead. Besides whenever a tests
interacts with a database it isn't a unittest anyway.

Better to use filenames which are commonly used in the python word then
custom ones.

* Tidy implementation of validator
* Tests improved and moved to tests/integration (as they call resolve
  internally)