code: function _end(debug) {
  
    const DEBUG_FIELDS = ['exitcode', 'output', 'stdout', 'stderr'];
  
    var summary = util.format(
      '%d test%s, %d success%s, %d failure%s',
      successes.length + failures.length,
      (successes.length + failures.length > 1) ? 's' : '',
      successes.length,
      (successes.length > 1) ? 'es' : '',
      failures.length,
      (failures.length > 1) ? 's' : ''
    );
  
    console.log('');
  
    if (failures.length === 0) {
      console.log(summary.green);
  
    } else {
  
      failures.forEach(function (failure) {
        console.error(util.format('' +
            '\n----------------' +
            '\n%s%s' +
            '\n%s',
          failure.test.description ? failure.test.description + '\n' : '',
          failure.test.file.cyan,
          failure.test.command));
  
        failure.errors.forEach(function (error) {
          console.error(error.red);
        });
  
        if (debug) {
          console.error('exec dir: %s\n'.grey, failure.test.dir);
  
          DEBUG_FIELDS.forEach(function (field) {
            if (failure.test[field]) {
              console.error('%s: %s\n'.grey, field, failure.result[field]);
            }
          });
        }
      });
  
      console.error(summary.red);
    }
  
  }
docs: End event handler, displays test summary and optional debug message.
  
  @param {Boolean} debug: if true, displays result output and exit code