Functional tests: add dependencies (browser-kit, css-selector and panther)

Documentation:

• https://symfony.com/doc/current/testing.html#your-first-functional-test
• https://symfony.com/doc/current/components/browser_kit.html
• https://symfony.com/doc/current/components/css_selector.html
• https://github.com/symfony/panther

Panther examples:

• https://www.christophe-meneses.fr/article/remplir-un-champ-de-formulaire-cache-avec-symfony-panther-php-webdriver
• https://symfony.com/blog/introducing-symfony-panther-a-browser-testing-and-web-scrapping-library-for-php

Installation:

composer require --dev symfony/browser-kit
composer require --dev symfony/css-selector
composer require --dev panther