Chore: Move model factories to the shared test layer (#14170)

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/test_models.py

@@ -2,8 +2,8 @@ import pytest
 
 from documents.models import Correspondent
 from documents.models import Document
-from documents.tests.factories import CorrespondentFactory
-from documents.tests.factories import DocumentFactory
+from paperless_testing.factories import CorrespondentFactory
+from paperless_testing.factories import DocumentFactory
 
 
 @pytest.mark.django_db