add missing fixture to MembersHelperTest

Test fails randomly.

<pre>
  1) Error:
MembersHelperTest#test_render_principals_for_new_members:
ActiveRecord::RecordInvalid: Validation failed: trackers is invalid, trackers is invalid, trackers is invalid
    test/object_helpers.rb:38:in `generate!'
    test/unit/helpers/members_helper_test.rb:29:in `test_render_principals_for_new_members'
</pre>

It can be reproduced on clean database and following change.

<pre>
-  fixtures :projects, :users, :members, :member_roles
+  fixtures :projects, :users, :members, :member_roles,
+           :trackers
</pre>

git-svn-id: http://svn.redmine.org/redmine/trunk@13749 e93f8b46-1217-0410-a6f0-8f06a7374b81