Bump test-unit dependency for Ruby 3.2 (or earlier) compat

Author: robinst

taglib-ruby.gemspec

@@ -27,7 +27,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'kramdown', '~> 2.3.0'
   s.add_development_dependency 'rake-compiler', '~> 0.9'
   s.add_development_dependency 'shoulda-context', '~> 1.0'
-  s.add_development_dependency 'test-unit', '~> 3.1'
+  s.add_development_dependency 'test-unit', '~> 3.5'
   s.add_development_dependency 'yard', '~> 0.9.26'
 
   s.extensions = [