Here this $client comes from symfony/browser-kit. (https://symfony.com/doc/current/testing.html#functional-tests)

Symfony-4.1-WebTestCase.php

<?php

namespace API\Tests\Controller;

use Symfony\Bundle\FrameworkBundle\Test\WebTestCase;
use Symfony\Component\HttpFoundation\Response;

class APIWebTestCase extends WebTestCase
{
    public function testEndpoint()
    {
        $client = static::createClient();
        $client->request('GET', '/api/endpoint');
        
        $this->assertSame(Response::HTTP_OK, $client->getResponse()->getStatusCode());
    }
}