Bump github.com/stretchr/testify from 1.8.0 to 1.8.1

Bumps [github.com/stretchr/testify](https://github.com/stretchr/testify) from 1.8.0 to 1.8.1.
- [Release notes](https://github.com/stretchr/testify/releases)
- [Commits](https://github.com/stretchr/testify/compare/v1.8.0...v1.8.1)

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

Signed-off-by: dependabot[bot] <support@github.com>

go.mod

@@ -5,5 +5,5 @@ go 1.16
 require (
 	github.com/gorilla/handlers v1.5.1
 	github.com/gorilla/mux v1.8.0
-	github.com/stretchr/testify v1.8.0
+	github.com/stretchr/testify v1.8.1
 )