Bump github.com/stretchr/testify from 1.6.1 to 1.8.1

Bumps [github.com/stretchr/testify](https://github.com/stretchr/testify) from 1.6.1 to 1.8.1.
- [Release notes](https://github.com/stretchr/testify/releases)
- [Commits](https://github.com/stretchr/testify/compare/v1.6.1...v1.8.1)

---
updated-dependencies:
- dependency-name: github.com/stretchr/testify
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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