Add extensive tests with node failures, packet loss, and leader reelection

[andreev-io]

Tests in this project serve two goals: testing functionality and offering usage examples. There's a lot of meaningful work that can be done by adding good tests to Little Raft.

Specifically, you'll want to test Little Raft functionality when replicas go down and up, when messages get lost on the network, and leaders get reelected. You might find it challenging to simulate nodes going up or down or getting connected and disconnected, but remember that in the tests the developer has control over how messages are passing between nodes -- emulating a disconnected node is as simple as not delivering any messages to it; simulating packet loss is as simple as dropping some packets randomly.

You could also add integration tests where replicas are actual separate processes communicating over the network. The possibilities are endless.