test_io.rb: fix r43164

* test/ruby/test_io.rb (test_pid_after_close_read): fix rr43164 for
  platforms other than Windows.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43174 b2dd03c8-39d4-4d8f-98ff-823fe69b080e