test_io.rb: avoid OOM
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61961 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
test_io.rb: avoid OOM
memory usage so the worker process will not die by OOM.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61961 b2dd03c8-39d4-4d8f-98ff-823fe69b080e