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/paperless_ai/tests/test_ai_classifier.py

@@ -12,9 +12,6 @@ from guardian.shortcuts import remove_perm
 
 from documents.models import Document
 from documents.search import TantivyBackend
-from documents.tests.factories import DocumentFactory
-from documents.tests.factories import TagFactory
-from documents.tests.factories import UserFactory
 from paperless.config import AIConfig
 from paperless_ai.ai_classifier import TAXONOMY_CANDIDATE_TOP_K
 from paperless_ai.ai_classifier import _fulltext_similar_documents
@@ -27,6 +24,9 @@ from paperless_ai.ai_classifier import get_taxonomy_context
 from paperless_ai.taxonomy import SimilarDocument
 from paperless_ai.taxonomy import TaxonomyCandidate
 from paperless_ai.taxonomy import TaxonomyCandidates
+from paperless_testing.factories import DocumentFactory
+from paperless_testing.factories import TagFactory
+from paperless_testing.factories import UserFactory
 
 
 @pytest.fixture