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.
maybe say the "first entry MUST have an index equal to the argument 'from'". It took me a second to realize from was an arg here.