test_io_m17n.rb: leaked FDs

* test/ruby/test_io_m17n.rb (test_ignored_encoding_option): fix
  leaked file descriptors by r47305.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47307 b2dd03c8-39d4-4d8f-98ff-823fe69b080e