Goals

Graphsync is big and has lots of tests. Those tests are not very easy to reach particularly because they involve a lot of tests on channels. This PR introduces Testify and a few utility functions to make the tests vastly more readable and clearer in intent.

Implementation

• Introduce testify, replace standard if -> t.Fatal pattern -- means test that correct conditions are met, rather than that the opposite (incorrect conditions trigger an error) -- makes for more clear, readable code
• Clean up error messages
• Refactor a few tests to switch to simpler table test form, BDD
• Introduce channel assertion utilities. Often we are testing for the fact that a value comes in on a channel before context cancellation, or one channel produces a value before others. We often need to read the value in the process. This produces a confusing pattern of select statements throughout the tests. While the channel assertions have to use some reflect fanciness internally to work for generic parameters (but with some type checking), it makes for vastly more readable assertions in the tests, making the intent clearer and the test way more concise