Seven etcd unit tests as boost tests.
* alejo/raft-tests-etcd-08-v4-communicate-v5:
raft: etcd unit tests: test proposal handling scenarios
raft: etcd unit tests: test old messages ignored
raft: etcd unit tests: test single node precandidate
raft: etcd unit tests: test dueling precandidates
raft: etcd unit tests: test dueling candidates
raft: etcd unit tests: test cannot commit without new term
raft: etcd unit tests: test single node commit
raft: etcd unit tests: update test_leader_election_overwrite_newer_logs
raft: etcd unit tests: fix test_progress_leader
raft: testing: log comparison helper functions
raft: testing: helper to make fsm candidate
raft: testing: expose log for test verification
raft: testing: use server_address_set
raft: testing: add prevote configuration
raft: testing: make become_follower() available for tests
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