ContainerAwareEventDispatcherTest depends on Symfony tests

Problem/Motivation

This is a bug report but I'm filing it against 8.4.x because it's not horribly urgent.

Drupal\Tests\Component\EventDispatcher\ContainerAwareEventDispatcherTest looks like this:

[..]
use Symfony\Component\EventDispatcher\Tests\ContainerAwareEventDispatcherTest as SymfonyContainerAwareEventDispatcherTest;
use Symfony\Component\EventDispatcher\Tests\TestEventListener;

/**
 * Unit tests for the ContainerAwareEventDispatcher.
 *
 * NOTE: 98% of this code is a literal copy of Symfony's emerging
 * CompiledEventDispatcherTest.
 *
 * This file does NOT follow Drupal coding standards, so as to simplify future
 * synchronizations.
 *
 * @see https://github.com/symfony/symfony/pull/12521
 *
 * @group EventDispatcher
 */
class ContainerAwareEventDispatcherTest extends SymfonyContainerAwareEventDispatcherTest

If we click through to that pull request: https://github.com/symfony/symfony/pull/12521 we see that it's been closed.

We should not depend on a test from a dependency, because it could move or change expectations.

A comment in #2874909-25: Update Symfony components to 3.3.* references this test.

Proposed resolution

Figure out what to do.

Remaining tasks

User interface changes

API changes

Data model changes