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.
If we do the assertion right now, it will break a lot of tests. We generate the log entries in the test without attaching the index field.