Extract an AbstractEventDispatcherTest from EventDispatcherTest and also use it in ContainerAwareEventDispatcherTest
minor #12131 Extract an AbstractEventDispatcherTest from EventDispatcherTest (znerol)

This PR was submitted for the master branch but it was merged into the 2.6 branch instead (closes #12131).

Discussion
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Extract an AbstractEventDispatcherTest from EventDispatcherTest

Allow third party implementations of `EventDispatcherInterface` to reuse the `EventDispatcherTest` unit tests. Also instantly expand test coverage of `ContainerAwareEventDispatcher` by rebasing it onto the new `AbstractEventDispatcherTest`.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
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8871368 Extract an AbstractEventDispatcherTest from EventDispatcherTest and also use it in ContainerAwareEventDispatcherTest