fix ProjectNestedSetTest fails randomly

<pre>
  1) Error:
ProjectNestedSetTest#test_adding_a_root_to_first_position_should_update_nested_set_order:
ActiveRecord::RecordInvalid: Validation failed: trackers is invalid, trackers is invalid, trackers is invalid
    test/unit/project_nested_set_test.rb:27:in `setup'
</pre>

You can reproduce with following change.

<pre>
 class ProjectNestedSetTest < ActiveSupport::TestCase
+  fixtures :trackers
</pre>

git-svn-id: http://svn.redmine.org/redmine/trunk@13730 e93f8b46-1217-0410-a6f0-8f06a7374b81