Next tosymfony/symfony#23311

Some people are used toset() a service in the container for mocking it in functional tests (seehttps://stackoverflow.com/a/19726963/4363634) but as of 3.3, setting/resetting a predefined service is deprecated.

The right solution is to load testing specific config (symfony/symfony#21533 (comment)).
I think we should add an example to the docs.