Update test-unit requirement from ~> 3.6.5 to ~> 3.6.7

Updates the requirements on [test-unit](https://github.com/test-unit/test-unit) to permit the latest version.
- [Release notes](https://github.com/test-unit/test-unit/releases)
- [Commits](test-unit/test-unit@3.6.5...3.6.7)

---
updated-dependencies:
- dependency-name: test-unit
  dependency-type: direct:development
...

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

Gemfile

@@ -25,6 +25,6 @@ group(:development) do
 end
 
 group(:test) do
-  gem('test-unit', '~> 3.6.5')
+  gem('test-unit', '~> 3.6.7')
   gem('warning', '~> 1.4.0')
 end