Chore: Add a shared test support layer (#14168)

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

src/paperless_testing/__init__.py

@@ -0,0 +1,5 @@
+"""Shared test support for every Paperless-ngx app.
+
+Fixtures live in ``src/conftest.py``; anything a test imports by name lives
+here. App-specific helpers stay in that app's ``tests`` package.
+"""