raft: fix raft_fsm_test flakiness
raft: fix raft_fsm_test flakiness

When election_threshold expires, the current node
can become a candidate, in which case it won't
switch back to follower state upon vote_request.

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