Make fixture for getting service with organisation

Saves repeatedly defining the same mock.

tests/conftest.py

@@ -2,7 +2,7 @@ import json
 import os
 from contextlib import contextmanager
 from datetime import date, datetime, timedelta
-from unittest.mock import Mock
+from unittest.mock import Mock, PropertyMock
 from uuid import UUID, uuid4
 
 import pytest
@@ -3138,6 +3138,18 @@ def mock_get_no_organisation_by_domain(mocker):
     )
 
 
+@pytest.fixture(scope='function')
+def mock_get_service_organisation(
+    mocker,
+    mock_get_organisation,
+):
+    return mocker.patch(
+        'app.models.service.Service.organisation_id',
+        new_callable=PropertyMock,
+        return_value=ORGANISATION_ID,
+    )
+
+
 @pytest.fixture(scope='function')
 def mock_update_service_organisation(mocker):
     def _update_service_organisation(service_id, organisation_id):