rec: Test that the "zero scope" option doesn't exceed the maximum payload size

rec: Test that the "zero scope" option doesn't exceed the maximum payload size

When use-incoming-edns-subnet is enabled, the query has an ECS option,
and the answer was not variable, we do return a 0-scoped ECS answer,
to make it possible to the client (dnsdist for example) to cache the
response and serve it to all clients.
Still we need to make sure that adding the 0-scoped ECS option does not
exceed the client EDNS UDP payload size.