Revert changes in #1913 and #2294 that required a stock record to have price_excl_tax set in order to report the product as available.

Add separate checks to the basket form/add logic that checks at the time of
adding a product to the basket whether a price exists, and report an error if it doesn't.

Fixes #2664.

- When tax is known but zero, line_tax should return Decimal(0).
- When tax is not known, line_tax should return None.

Fixes #2395.

In Python 2, adding None to the basket total worked, but in Python 3 this raises a TypeError. This fixes the handling of totals to distinguish between a zero price and a non-existent price.

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.

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.

str.decode doesn't exist in Python 3 anymore, solve it using the
smart_text function from django.utils.encoding.

This fixes #1708

- Restructure into unit and integration tests.
- Add more tests for the various basket properties.