Update test-unit requirement from ~> 3.6.2 to ~> 3.6.3 (#275)

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@3.6.2...3.6.3)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Gemfile

@@ -12,7 +12,7 @@ group :development do
 end
 
 group :test do
-  gem 'test-unit', '~> 3.6.2'
+  gem 'test-unit', '~> 3.6.3'
   gem 'test-unit-ruby-core', '~> 1.0'
   gem 'warning', '~> 1.4.0'