Fix failing test with new Python version

This test is now failing due to the combination of both Werkzeug having
had a major version bump in a previous PR and the Python version
changing. Calling `current_service` is now giving an error when trying
to get the `service` from the `_request_ctx_stack` because
`_request_ctx_stack` has no `service` attribute. There was no error
before, so it appears some underlying behaviour has changed. I've fixed
the test by changing the setup, but there is no effect on production
code - when the app is running `load_service_before_request` is called
before every request.

tests/app/test_template_previews.py

@@ -5,6 +5,7 @@ from unittest.mock import Mock
 import pytest
 from notifications_utils.template import LetterPreviewTemplate
 
+from app import load_service_before_request
 from app.template_previews import (
     TemplatePreview,
     get_page_count_for_letter,
@@ -58,6 +59,10 @@ def test_from_database_object_makes_request(
     expected_filename,
     mock_get_service_letter_template
 ):
+    # This test is calling `current_service` outside a Flask endpoint, so we need to make sure
+    # `service` is in the `_request_ctx_stack` to avoid an error
+    load_service_before_request()
+
     resp = Mock(content='a', status_code='b', headers={'c': 'd'})
     request_mock = mocker.patch('app.template_previews.requests.post', return_value=resp)
     mocker.patch('app.template_previews.current_service', letter_branding=letter_branding)