The initial check if the voucher is available was done before creating
the order. This change adds a validation step during the order creation
within an atomic() block.

See #2101

Most real-world instances will use postgresql instead of sqlite so it's
better to use postgres for the tests. This also allows us to easily add
tests for concurrency issues which is not supported by sqlite

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`

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.

The tests were failing on python 2 due to the fact that the __init__.py
wasn't written so it wasn't a valid module. With this commit we now
also fully leverage pytest functionality to handle temporary paths for
us.

This fixes a DeprecationWarning

Latter one is deprecated

A dict_values object was returned instead of a list. See #2223

Future tasks is to dig into self.assertRedirectsTo wouldn't work with the test
as it stands