add missing fixture to test/unit/enabled_module_test.rb

Test fails randomly.

<pre>
  1) Error:
EnabledModuleTest#test_enabling_wiki_should_create_a_wiki:
ActiveRecord::RecordInvalid: Validation failed: trackers is invalid, trackers is invalid, trackers is invalid
    test/unit/enabled_module_test.rb:25:in `test_enabling_wiki_should_create_a_wiki'
</pre>

It can be reproduced on clean db and following change.

<pre>
-  fixtures :projects, :wikis
+  fixtures :projects, :trackers, :wikis
</pre>

git-svn-id: http://svn.redmine.org/redmine/trunk@13571 e93f8b46-1217-0410-a6f0-8f06a7374b81