Improve coverage of suspend service tests
Improve coverage of suspend service tests

Previously these only tested with a Platform Admin user, but service
admins can suspend a service too. I've rewritten the tests to match
the 'archive_service' ones, which use the client_request fixture to
make changing the user easier.

Note that the return value of the service API client wasn't used for
anything, so it's safe to remove it from the mock.

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin