raft: etcd unit tests: test cannot commit without new term
raft: etcd unit tests: test cannot commit without new term

TestCannotCommitWithoutNewTermEntry tests the entries cannot be
committed when leader changes, no new proposal comes in and ChangeTerm
proposal is filtered.

NOTE: this doesn't check committed but it's implicit for next round;
      this could also use communicate() providing committed output map

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

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