test: rename swim_cluster_node to swim_cluster_member

There are two different structures - public struct swim_member
exposed by SWIM API, and struct swim_node defined and used inside
tests. Before this patch swim_cluster_node() was returning struct
swim_member, just historically. But more and more places appear
where it is wanted to safely take struct swim_node, not
swim_member, with an appropriate assertion on an invalid index.

This patch renames swim_cluster_node() to swim_cluster_member(),
and introduces new swim_cluster_node() returning swim_node.