Add test for FindPeer

[nonsense]

This should be done as part of the simple-fetchers branch.

The functionality that decides who the next eligible peer is based on a given Kademlia table is not tested thoroughly and we should add tests to it.

// FindPeer is returning the closest peer from Kademlia that a chunk
// request hasn't already been sent to
func (d *Delivery) FindPeer(ctx context.Context, req *storage.Request) (*Peer, error) {

Feel free to modify the interface of this method and suggest alternatives, but ultimately we should have good coverage on the algorithm that decides who is the next selected Peer for a given request.