Somehow related to #20493 (cc @nicolas-grekas).

Symfony project treats (which is good) it tests as internal code that should serve only this repo purposes.
It's not common, but I do saw very edge case when someone actually extend concrete Foo...TestCase of Symfony unittests. I would like to propose marking all utests as @internal and @final (or maybe even final class on class-syntax level, not docs) - to prevent that kind of situations.
In the case I describe the exotic project tests crashed after updating Symfony becouse of BC breaks on Foo...TestCase class.

If agreed, I could prepare a PR.