Chore: Add a shared test support layer

All four Django apps have test code in common, but the only place to put it
was the documents app's own tests package, so paperless, paperless_ai and
paperless_mail each reached across an app boundary to import helpers and
relied on fixtures that were only defined for the documents test path.

This adds a root src/conftest.py holding the fixtures every app needs and an
new src/paperless_testing package for shared helpers a test names

src/documents/tests/conftest.py

@@ -7,8 +7,6 @@ from typing import TYPE_CHECKING
 import filelock
 import pytest
 from django.contrib.auth import get_user_model
-from django.contrib.contenttypes.models import ContentType
-from guardian.shortcuts import clear_ct_cache
 from pytest_django.fixtures import Settings
 from rest_framework.test import APIClient
 
@@ -154,30 +152,6 @@ def user_client(rest_api_client: APIClient, regular_user: UserModelT) -> APIClie
     return rest_api_client
 
 
-@pytest.fixture(autouse=True)
-def _clear_content_type_caches() -> None:
-    """Clear Django's ContentType cache and guardian's lru_cache before each test.
-
-    Tests that delete and reinsert ContentType/Permission rows (e.g. the
-    importer) corrupt both caches. Without this fixture a subsequent test on
-    the same xdist worker sees stale ContentType objects and guardian raises
-    MixedContentTypeError.
-    """
-    ContentType.objects.clear_cache()
-    clear_ct_cache()
-
-
-@pytest.fixture(scope="session", autouse=True)
-def faker_session_locale():
-    """Set Faker locale for reproducibility."""
-    return "en_US"
-
-
-@pytest.fixture(scope="session", autouse=True)
-def faker_seed():
-    return 12345
-
-
 @pytest.fixture
 def indexed_document(_search_index: None) -> "Document":
     """One searchable document, for tests about what the search endpoint