As in `test_model.py` -> `test_models.py`

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.

Before this change, the type of a bankcard would be clobbered by saving
and reloading.

Fixes #1486, #1487

Write a few more tests at the same time.

Useful in templates where the bankcard form is rendered.  Eg:

    {{ bankcard_form.bankcard.obfuscated_number }}