Incorrect use of clean_returnvalue in forum externallib tests

Example:

        $discussions = mod_forum_external::get_forum_discussions(array($forum1->id, $forum2->id));

        external_api::clean_returnvalue(mod_forum_external::get_forum_discussions_returns(), $discussions);

The clean_returnvalue returns the result cleaned, in this case the function return should be assigned to $discussions

Note that this issue is different than MDL-45131 (here, the functions were missing the clean_returnvalue)