Fuel\Validation\Validator::addCustomRule PHP Method

    public function addCustomRule($name, $class)
    {
        $this->customRules[$name] = $class;
        return $this;
    }

 public function testCreateRuleInstanceWithGlobalMessage()
 {
     $message = 'Test message';
     $ruleName = 'test';
     $class = '\\DummyAbstractRule';
     $this->object->setGlobalMessage($ruleName, $message);
     $this->object->addCustomRule($ruleName, $class);
     $instance = $this->object->createRuleInstance($ruleName);
     $this->assertEquals($message, $instance->getMessage());
 }