test/ruby/test_io.rb: avoid EBADF

* test/ruby/test_io.rb (test_readpartial_locktmp): avoid EBADF
  [ruby-core:64773] [ruby-core:64775]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47422 b2dd03c8-39d4-4d8f-98ff-823fe69b080e