Add kernel tests for event system

What else should we cover?

Believe this added test coverage looks good. Marking for committers.

Good catch on seeing this.

Thanks @smustgrave for rtbc!
But, I really want these to be somewhat "complete". I would like to add anything that makes sense to add before we merge this.

Are you sure it is enough, or can you think of anything we should add?

Background: We want to catch regressions from replacing the event subscriber in 📌 Replace ContainerAwareEventDispatcher with Symfony EventDispatcher Fixed .

To me it looks good. But maybe should get a second +1

Perhaps we should cover different return value array shapes from the event subscriber.
Given that we would replace the compiler pass, we want to be sure all of that is still supported.

I think now it covers all of RegisterEventSubscribersPass.

There is some logic in ContainerAwareEventDispatcher that is not covered by this new kernel test, but it is already covered sufficiently by the unit test.
We just need to watch out when we replace the ContainerAwareEventDispatcher with the symfony event dispatcher, that the behavior covered in the unit test equally applies to the symfony event dispatcher.

Moving to committers queue.

I'm triaging RTBC issues → . I read the IS, the comments and skimmed the MR. I didn't find any unanswered questions.

Leaving at RTBC.

The Needs Review Queue Bot → tested this issue. It no longer applies to Drupal core. Therefore, this issue status is now "Needs work".

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