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

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@7617 e93f8b46-1217-0410-a6f0-8f06a7374b81