test: allow to select group and replicaset in config builder

It's now possible to select a custom replicaset for following calls,
such as :add_instance(), in the config builder with the new methods
:use_group() and :use_replicaset(). This is necessary for creating
clusters spanning multiple replicasets.

NO_DOC=testing helper update
NO_CHANGELOG=see NO_DOC

Co-authored-by: Vladimir Davydov <vdavydov@tarantool.org>