view change test automation

[LeoHChen]

We shall add view change test automation in the test package for the regression test.

You may refer to the following files to check out the flow. Basically, iterate all the nodes to find out the leader, and then gracefully kill the leader. After 60 seconds, the view change is expected to be finished and then restore the original node.

https://github.com/harmony-one/harmony/pull/3408/files

Some enhancement could be to check the log or RPC to make sure the view change happened and the consensus was ongoing after the view change.