raft: add test for findConflict

/cc @unihorn @jonboulle @barakmich @bdarnell 
I added extensive comments to the tested function. So this should be easy to review even if you are not very familiar with raft. It is super helpful to have more eyes on raft.
i think it is too long to read.. what about `findConflict returns the first place in the given ents that is different from log entries that starts from the given from, or it will return 0 if it cannot find conflict.`