test_stringio.rb: reduce retry count
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55544 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
test_stringio.rb: reduce retry count
retry count to get rid of timeout on some platforms.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55544 b2dd03c8-39d4-4d8f-98ff-823fe69b080e