Resolve #18 *TST? should return 0 as OK

 - it should be completely customizable, what return value is return
   from the callback function
 - Returning 0 from callback means "test was OK"
 - Other number means, that some error bits are set
 - Type of callback is still wrong - it should be int and not
   scpi_result_t but it will change API. It will be part of next version