raft: Add test that triggers etcd/raft panic
raft: Add test that triggers etcd/raft panic

This test panics without the newly-added protection against having the
same manager call Join twice.

Signed-off-by: Aaron Lehmann <8ecfc6017a87905413dcd7d63696a2a4c351b604@docker.com>

docker/swarmkit,sungwonh/swarmkit,cheyang/swarmkit,sungwonh/swarmkit,dperny/swarmkit-1,allencloud/swarmkit,fate-grand-order/swarmkit,jlhawn/swarmkit,AliyunContainerService/swarmkit,RenaudWasTaken/swarmkit,RenaudWasTaken/swarmkit,fate-grand-order/swarmkit,cheyang/swarmkit,elifa/swarmkit,Noyan759/swarmkit,deljuven/swarmkit,aaronlehmann/swarmkit,jlhawn/swarmkit,alyyousuf7/swarmkit,diogomonica/swarmkit,allencloud/swarmkit,dperny/swarmkit-1,diogomonica/swarmkit,deljuven/swarmkit,elifa/swarmkit,AliyunContainerService/swarmkit,docker/swarmkit,alyyousuf7/swarmkit,aaronlehmann/swarmkit,Noyan759/swarmkit,yongtang/swarmkit,yongtang/swarmkit