test_dir.rb: fix system dependent test

* test/ruby/test_dir.rb (test_symlinks_not_resolved): fix system
  dependent test which has no meanings unless symlink is available.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44223 b2dd03c8-39d4-4d8f-98ff-823fe69b080e