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.
could you write the section of raft paper that defines this here too? It could help to read it IMO.

The comment doesn't explain why it says it is more up-to-date if (index, term) pairs are the same.