Skip forum search tests on dev as no results are guaranteed to exist

This addresses the failures as seen at [1] and [2].

[1] http://qa-selenium.mv.mozilla.com:8080/view/SUMO/job/sumo.dev/5703/testReport/tests.desktop.test_search/TestSearch/test_user_flow_to_forum_post/
[2] http://qa-selenium.mv.mozilla.com:8080/view/SUMO/job/sumo.dev/5703/testReport/tests.desktop.test_search/TestSearch/test_no_query_adv_forum_search/