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/paperless_mail/tests/test_mail.py

@@ -28,7 +28,6 @@ from rest_framework.test import APITestCase
 
 from documents.models import Correspondent
 from documents.models import MatchingModel
-from documents.tests.factories import CorrespondentFactory
 from documents.tests.utils import DirectoriesMixin
 from documents.tests.utils import FileSystemAssertsMixin
 from paperless_mail import tasks
@@ -43,6 +42,7 @@ from paperless_mail.models import MailRule
 from paperless_mail.models import ProcessedMail
 from paperless_mail.tests.factories import MailAccountFactory
 from paperless_mail.tests.factories import MailRuleFactory
+from paperless_testing.factories import CorrespondentFactory
 
 
 @dataclasses.dataclass