The pk kwarg on views is a string type. The message format string was expecting a numeric value. This updates that format string to expect a string value, which will always display correctly.

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

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

Replace 'assert' with 'assert*' method.

Convert tests.unit.forms.widget_tests to use TestCase class.

Convert bankcard tests to use TestCase.

Convert tests.unit.logging_tests to use TestCase.

This also fixes the `end_datetime` value set by the VoucherFactory