Proposal to use stretchr/testify in tests

[kostyantyn]

Motivation of using this package in tests is to have a nice error message and write less code https://github.com/stretchr/testify

[joshuap]

:+1:

[mperham]

Longtime fan of the assert package here.

[joshuap]

@mperham gtk, thanks! I was hoping someone would suggest a good testing package; I had looked at testify initially but decided just to go with the more verbose built-in API until we needed the dependency. I'm all for cleaning up the assertions in the current suite. @kostyantyn feel free to take a stab at it if you want.