Remove deprecated lookup of guest_user from application

Lookup of guest user from application is due for deprecation as
it can be easily obtained from the current website and could differ
from website to website.
Migrate to new 2.4 testing style

As part of the inclusion of nereid into tryton core
the test suite and related capabilities were updated to
use the standard tryton test fixtures.

This patch follows the changes other related modules have had
in the path and updates testing to 2.4.0.8 style.