Allow test DI-extenions that depends on another extensions and config

[Koc]

See dustin10/VichUploaderBundle#746 for use case. I need load TwigExtension with his own config.

But I cann't just add new TwigExtenstion() to getContainerExtensions method. Can we make it more user-friendly? Allow pass to load method array with keys like twig => [/* twig config */], vichuploader => [/* vichuploader config */]?