Issue #757: use stretchr/testify/require

[magiconair]

stretchr/testify is more strict in equality testing and provides more useful debugging output.

In a first step I've replaced all usages of verify.Values with testify and found two glitches in the variant tests.

In later steps I'll go through the if got, want pattern and gradually replace them with require as well.

[kung-foo]

You've put this in the v0.6.2 milestone, but it looks like there are changes to the other PRs that are targeting v0.6.1 that also have testify updates. Do we need to have all of them target one milestone?

[magiconair]

I changed the CodecTest to stretchr/testify in a previous commit since it allowed me to find a failing test with the nil slices. We already have two different test patterns with got, want and verify.Values. I'm trying to switch this all to require so that there is only one.

We don't need to have them all in one milestone and do them one step at a time as long as they're all converted at some point.