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.
It isn't clear from the comment what happens when the two logs are equal nor whether it returns true from the point of view of the args or l.