Evaluate moving unit tests to stretchr/testify

[danehans]

Please describe the problem you have Evaluate whether standardizing on the stretchr/testify framework for unit tests is worth adding as an external dependency.

/cc @jpeach

[youngnick]

We use it in mainline Contour, so I'm supportive.

[jpeach]

We use it in mainline Contour, so I'm supportive.

Yeh we don't use the whole framework, but the require and assert packages are pretty handy and save a bunch of tedious test code.