There is another solution posted in the Github discussion that doesn't require rewriting the loginUser

(Taken from https://github.com/symfony/symfony/discussions/46961#discussioncomment-4573371 )

<?php

namespace App\Tests;

use Symfony\Bundle\FrameworkBundle\KernelBrowser;
use Symfony\Component\BrowserKit\Cookie;
use Symfony\Component\HttpFoundation\Session\Session;
use Symfony\Component\HttpFoundation\Session\Storage\MockFileSessionStorage;
use Symfony\Component\Security\Csrf\TokenGenerator\TokenGeneratorInterface;
use Symfony\Component\Security\Csrf\TokenStorage\SessionTokenStorage;

trait SessionHelper
{
    public function getSession(KernelBrowser $client): Session
    {
        $cookie = $client->getCookieJar()->get('MOCKSESSID');

        // create a new session object
        $container = static::getContainer();
        $session = $container->get('session.factory')->createSession();

        if ($cookie) {
            // get the session id from the session cookie if it exists
            $session->setId($cookie->getValue());
            $session->start();
        } else {
            // or create a new session id and a session cookie
            $session->start();
            $session->save();

            $sessionCookie = new Cookie(
                $session->getName(),
                $session->getId(),
                null,
                null,
                'localhost',
            );
            $client->getCookieJar()->set($sessionCookie);
        }

        return $session;
    }

    public function generateCsrfToken(KernelBrowser $client, string $tokenId): string
    {
        $session = $this->getSession($client);
        $container = static::getContainer();
        $tokenGenerator = $container->get('security.csrf.token_generator');
        $csrfToken = $tokenGenerator->generateToken();
        $session->set(SessionTokenStorage::SESSION_NAMESPACE . "/{$tokenId}", $csrfToken);
        $session->save();
        return $csrfToken;
    }
}

Used like this:

<?php

namespace App\Tests\Controller;

use App\Tests\SessionHelper;
use Symfony\Bundle\FrameworkBundle\Test\WebTestCase;

class SessionControllerTest extends WebTestCase
{
    use SessionHelper;

    public function testSomething(): void
    {
        $client = static::createClient();

        $client->request('POST', '/something', [
            '_csrf_token' => $this->generateCsrfToken($client, 'expected token id'),
        ]);

        // assert something
    }
}