[stringio] Add test-unit in the development dependencies

https://github.com/ruby/stringio/commit/de010fc0e9

ext/stringio/stringio.gemspec

@@ -30,4 +30,5 @@ Gem::Specification.new do |s|
   # s.signing_key = File.expand_path("~/.ssh/gem-private_key.pem") if $0 =~ /gem\z/
 
   s.add_development_dependency 'rake-compiler'
+  s.add_development_dependency 'test-unit'
 end