Mercure and WebSocket testing: allow to create several browser instances

[dunglas]

Easily test applications using Mercure, WebSocket and similar technologies with Panther!

This PR introduces a new method to create additional extra, isolated, browsers; that can interact with the primary one. For instance, it can be useful to test a chat application:

use Symfony\Component\Panther\PantherTestCase;

class ChatTest extends PantherTestCase
{
    public function testMultiClient(): void
    {

        $client1 = self::createPantherClient()
        $client1->request('GET', '/chat'); 
        // Connect a 2nd user, using an isolated browser
        $crawler2 = self::createAdditionalPantherClient()->request('GET', '/chat');

        // Say hi
        $crawler2->submitForm('Post message', ['message' => 'Hi folks']);

        // Wait for the message to be propagated
        $client1->waitFor('.message');
        $this->assertSelectorTextContains('.message', 'Hi folks'); // Assertions are always executed in the PRIMARY browser; but you can use the crawler of the other browsers directly if needed
    }
}

[dunglas]

Merged in f5f377f. It looks like a GitHub bug occurred, that prevents this PR to be marked as merged.