config/test: update cluster.lua helper

Before this patch, there was no way to configure empty replicaset
with cluster helper, while this is a valid mistake we should verify.

This patch also adds verification on cluster:new() and
cluster.startup_error() to forbid passing intermediate
config representation that is return by cbuilder.new().

Part of #9895

NO_DOC=test helper update
NO_CHANGELOG=see NO_DOC
NO_TEST=see NO_DOC