testing framework

[WillMatthews]

This is a bit of an opinionated change (feel free to completely reject), but I think using testify may help with test quality (in terms of readability, DRY, lines of code).

A drawback and potential reason for rejection is that this PR adds a dependency.

This PR does the following:

• Makes all tests use testify's require
• Fixes a few comments, and removes a few unnecessary ones
• Makes the testing helper function toPtr a generic

[liushuangls]

I prefer to keep dependencies to a minimum, so perhaps we can add some helper functions to the checks package to optimize the test code.

[WillMatthews]

sounds sensible to me! We only use a tiny part of the require package, so I think using "a little copying is better than a little dependency" is a fair Go Proverb to rest on here. (I'll have a try at this over the next few days).