Cannot test Swift_Mailer because service already initialized or test. alias ignored

Question

I already found few informations like Symfony2 access private services in tests or Replace Symfony service in tests for php 7.2

But I dont know why its not working.

I have an autowiring service.

class MailService {

    public function __construct(\Swift_Mailer $mailer, Environment $twig)
    {
        $this->mailer = $mailer;
        $this->twig = $twig;
    }

}

config/services_test.yaml

services:
    # default configuration for services in *this* file
    _defaults:
        #autowire: true      # Automatically injects dependencies in your services.
        #autoconfigure: true # Automatically registers your services as commands, event subscribers, etc.
        public: true

    test.swiftmailer.transport: '@Swift_Mailer'

And when I try to replace.

        $mailer = $this->getMockBuilder(\Swift_Mailer::class)
            ->disableOriginalConstructor()
            ->getMock();
        $mailer->expects($this->once())->method('send')->willReturn(1);
        self::$container->set('swiftmailer.mailer.default', $mailer);
        self::$container->set('swiftmailer.default', $mailer);
        self::$container->set('swiftmailer.mailers', [$mailer]);

But I dont know why its not working.

Any ideas? :)