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/search/test_highlight_query_guard.py

@@ -29,7 +29,7 @@ from rest_framework import status
 
 from documents.search._backend import SearchMode
 from documents.search._query import parse_simple_text_highlight_query
-from documents.tests.factories import DocumentFactory
+from paperless_testing.factories import DocumentFactory
 
 if TYPE_CHECKING:
     from rest_framework.test import APIClient