Make test use new fixture
Make test use new fixture

The `client_request` fixture means the tests take fewer lines of code
and also checks some extra things (eg page titles matching <h1>s).

Changing it in this commit, independently of functional changes to this
test.

gov-cjwaszczuk/notifications-admin,alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin,gov-cjwaszczuk/notifications-admin