Fixed a settings manipulation that was breaking other tests.

The manipulation of ASKBOT_EXTRA_SKINS was breaking widget_tests and
post_model_tests when running the full askbot test suite.

If the settings attribute ASKBOT_EXTRA_SKINS wasn't set before the test running, it would be initialized with value None on the tearDown. This would break subsequent tests that would test for the existance of ASKBOT_EXTRA_SKINS.