Symfony version(s) affected: 4.1.0

Description
Service test.service_container does not respect the output of kernel's getContainerBaseClass method.

Therefore, when FrameworkBundle's Client returns it on getContainer, we receive an instance of TestContainer instead of SomeCustomContainer. This makes it impossible to call a custom method added to the container (in our case - https://github.com/PolishSymfonyCommunity/SymfonyMockerContainer).

How to reproduce
Either:

• Define custom container base class and call a method on custom container coming from FrameworkBundle's Client.
• Install Sylius v1.2.0-BETA and try to run phpunit tests.

Possible solution
Adding a __call method on TestContainer which passes unknown calls to TestContainer's public container.

Additional context
I'm up to making a PR fixing it (using the solution proposed above), but not before Wednesday, and I don't mind anyone fixing it before that :)