CHORE(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

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

Refs: #7