fix specs for ReportController#replace_right_cell
fix specs for ReportController#replace_right_cell

   * moved the common code to `before`
   * removed obsolete x_build_dyna_tree mention
   * don't overwrite @sb by calling contoller.instance_variable_set twice
   * pre-initialize all the trees - otherwise tree_exists? won't find them
   * also test `build_reports_tree`, especially in the one test that was supposed to check it gets re-rendered when explicitly called with `:replace_trees`
   * add a spec testing only existing trees get rendered

ManageIQ/manageiq-ui-classic,ManageIQ/manageiq-ui-classic,ManageIQ/manageiq-ui-classic,ManageIQ/manageiq-ui-classic