fix: bug in conf_test when demoting excess voters

The bug was introduced when I changed the behaviour of conf_change in
regard to instances with target_state Expelled. As a result we would
sometimes arbitrarily demote healthy voters in presence of degenerate
ones.

For example we could have this situation:

instance i1: raft_id=1, target_state=Online, raft_configuration=voter
instance i2: raft_id=2, target_state=Expelled, raft_configuration=voter (!)
instance i3: raft_id=3, target_state=Offline, raft_configuration=voter (!)
instance i4: raft_id=4, target_state=Online, raft_configuration=learner (!)
instance i5: raft_id=5, target_state=Online, raft_configuration=learner (!)