More complete clamdcscan tests
 * construct our own Qpsmtpd::Transaction object for testing, so we're sure
   to have a pristine state
 * Move some logic into a should_scan() sub, to help separate scan_all from
   is_not_multipart (since the multipart state of a message has nothing to
   do with any configuration parameters)
Style cleanup

 * no more 'return ()'
 * Explicit 'return 0'