Rails3: test: replace deprecated errors.on at test_creating_a_child_in_different_project_should_not_validate of unit/issue_nested_set_test.rb

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@7594 e93f8b46-1217-0410-a6f0-8f06a7374b81