raft: replication test: test_server inside raft_cluster
raft: replication test: test_server inside raft_cluster

Since there are no more external users of test_server, move it to
raft_cluster and remove member access operator.

Signed-off-by: Alejo Sanchez <004be06a28e38b437b038f9b5c0b9f5758b792e9@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla