deps: fix dependency cycle with testify (#124)

[dolmen]

Summary

In go.mod exclude the previous of testify to break the dependency cycle. Fixes stretchr/objx#124 and stretchr/testify#1292 (once testify upgrades).

$ go mod edit -exclude=github.com/stretchr/testify@v1.8.0
$ go mod tidy

Checklist

• [x] Tests are passing: task test
• [x] Code style is correct: task lint (well, see #139)

[dolmen]

Thanks for the merge and the release. As I am now a co-maintainer of testify I'll quickly propose the fix for testify side.