test_dir_m17n.rb: fixed encoding file system on OS X

* test/ruby/test_dir_m17n.rb (test_filename_as_bytes_extutf8),
  (test_filename_bytes_euc_jp, test_filename_euc_jp),
  (test_filename_ext_euc_jp_and_int_utf_8): Mac OS X also has fixed
  encoding file system.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42549 b2dd03c8-39d4-4d8f-98ff-823fe69b080e