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        | -