raft: etcd unit tests: test single node precandidate
raft: etcd unit tests: test single node precandidate

TestSingleNodePreCandidate
Checks a single node configuration with precandidate on works to
automatically elect the node.

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

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