handle testify output

[bhcleek]

I'm not sure if we want to do this or not, but the fix was relatively simple... testify is its own beast, though, and I'm reluctant to add official support for it.

Fixes #1636

[fatih]

This came up before and I've said 👎 Reason is that maintaining third party compatibility is not worth it. We should aim to support for Go only. This should go into a plugin called vim-go-testify or something alike. We use third party tools, but those provide a feature that we control, here someone decides to use on their own and we're trying to fix it. There was also another PR that would add support for a third party templating library, but I've also rejected that as well due the same reason.

I know you want to be helpful, but this is not what vim-go should do going forward.

[ernesto-jimenez]

I believe this should address the issue: stretchr/testify#554