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.

Issue #1503 was rather embarrassing and highlighted it's about time we
have tests for the form. This vets the logic in the PhoneNumberMixin as
well.