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.
This has a historic reason. The entry does not have an index field previously. We added a index field for the wal and other kinds of validation we want to do. We do not actually use the index field in this function right now. But we will start to clean them up and add the validation soon. You should expect the assertion to be added.