All four Django apps have test code in common, but the only place to put it
was the documents app's own tests package, so paperless, paperless_ai and
paperless_mail each reached across an app boundary to import helpers and
relied on fixtures that were only defined for the documents test path.
This adds a root src/conftest.py holding the fixtures every app needs and an
new src/paperless_testing package for shared helpers a test names
