Chore: convert test_signals.py from TestCase to pytest style

22 near-duplicate test methods. Converts to plain pytest classes to enable
pytest.mark.parametrize, which collapses those into 9 test functions covering 23 cases,
with mocker/caplog/settings fixtures replacing unittest.mock/assertLogs/override_settings.

The role-sync scenarios (superuser/staff group sync, in various combinations)
share the same setup and assertions, so they're merged into one parametrize
table.

src/paperless_testing/factories.py

@@ -6,6 +6,7 @@ from __future__ import annotations
 
 import factory
 from django.contrib.auth import get_user_model
+from django.contrib.auth.models import Group
 from factory.django import DjangoModelFactory
 
 from documents.models import Correspondent
@@ -71,6 +72,13 @@ class DocumentFactory(DjangoModelFactory[Document]):
     storage_path = None
 
 
+class GroupFactory(DjangoModelFactory[Group]):
+    class Meta:
+        model = Group
+
+    name = factory.Sequence(lambda n: f"group{n}")
+
+
 class UserFactory(DjangoModelFactory[UserModelT]):
     class Meta:
         model = UserModelT