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.
An entry is considered to be conflicting if it has the same index but a different term. The first given entry MUST have an index equal to `from`.