Add test coverage for setting service permission
Add test coverage for setting service permission

Spotted that we aren't testing all our permission types here so added
this one in.

It includes the TODO for allowing the API to give a service the
broadcast permission. We don't want this to happen, we want them to use
the `set_as_broadcast_service` route instead. We will probably get away
with it for the moment for it would be tighter validation we should add
to reduce the risk of letting a service get in a dodgy state.

alphagov/notifications-api,alphagov/notifications-api