Chore: Move model factories to the shared test layer

The model factories lived in the documents test package, but three other
apps needed them. The AI, mail and testing suites all reached across an app
boundary to import from documents.tests.factories, which made a private test
package into a shared dependency.

The factories now live in the shared testing package, where cross-app use is
the intended use.

src/documents/tests/conftest.py

@@ -10,7 +10,7 @@ from django.contrib.auth import get_user_model
 from pytest_django.fixtures import Settings
 from rest_framework.test import APIClient
 
-from documents.tests.factories import DocumentFactory
+from paperless_testing.factories import DocumentFactory
 
 UserModelT = get_user_model()