Chore: Move unittest directory setup onto the shared fixture

The unittest side of the suite built its temp directory tree with
tempfile.mkdtemp and a manually enabled override_settings, cleaned up only if
tearDown ran. That is now gone. DirectoriesMixin lives alongside the layout it
bridges and does nothing but hand the paperless_dirs fixture to TestCase
subclasses as self.dirs, so both halves of the suite get the same twelve
settings, the same directory shapes and cleanup owned by tmp_path.

The mixin moves to paperless_testing.dirs rather than staying in the documents
test utilities, because modules in paperless and paperless_mail import it
across the app boundary. The thirty-eight consuming modules change only their
import line; self.dirs.scratch_dir and its siblings keep working.

src/documents/tests/test_api_custom_fields.py

@@ -13,7 +13,7 @@ from rest_framework.test import APITestCase
 from documents.models import CustomField
 from documents.models import CustomFieldInstance
 from documents.models import Document
-from documents.tests.utils import DirectoriesMixin
+from paperless_testing.dirs import DirectoriesMixin
 
 
 class TestCustomFieldsAPI(DirectoriesMixin, APITestCase):