Implements tests

[lenybernard]

• [x] Dependency injection (parameters and services drived by configuration)
• [x] Helper (each shortcut method will correctly add what is expected in the correct FlashBag)
• [x] SessionHandler (what is in session is handled as well to be transformed into some template)
• [ ] Listener (on kernel.response, we correctly inject the templates into the page to show alerts ex. https://github.com/symfony/web-profiler-bundle/blob/2.8/Tests/EventListener/WebDebugToolbarListenerTest.php)
• [ ] TwigExtension (we can render alerts directly in twig thanks to this extension)