Bump github.com/stretchr/testify from 1.8.4 to 1.9.0 (closes #9)

Bumps [github.com/stretchr/testify](https://github.com/stretchr/testify) from 1.8.4 to 1.9.0.
- [Release notes](https://github.com/stretchr/testify/releases)
- [Commits](https://github.com/stretchr/testify/compare/v1.8.4...v1.9.0)

Change-Id: Ia3510de8542c7dd7292fd0606cfec8b79e4ac4a7

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

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

Change-Id: Iecab6ffd6ae157bdaa43cdd553abebc0ac2f1c9f
diff --git a/go.mod b/go.mod
index 39e8550..69f4030 100644
--- a/go.mod
+++ b/go.mod
@@ -4,5 +4,5 @@
 
 require (
 	github.com/alecthomas/kong v0.8.1
-	github.com/stretchr/testify v1.8.4
+	github.com/stretchr/testify v1.9.0
 )