Update test-unit requirement from ~> 3.3.0 to ~> 3.4.0 (#222)

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@3.3.0...3.4.0)

Signed-off-by: dependabot[bot] <[email protected]>

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

Author: dependabot[bot]

pygments.rb.gemspec

@@ -18,7 +18,7 @@ Gem::Specification.new do |s|
 
   s.add_development_dependency 'rake', '~> 13.0.0'
   s.add_development_dependency 'rubocop', '~> 0.81.0'
-  s.add_development_dependency 'test-unit', '~> 3.3.0'
+  s.add_development_dependency 'test-unit', '~> 3.4.0'
 
   # s.extensions = ['ext/extconf.rb']
   s.require_paths = ['lib']