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.
the purpose of the comment has nothing to do with raft. 
it simply explains what this functions intends to do. so the people who are not familiar with raft can see if the implementation is correct.