Add ActorBase lifecycle flow unit tests

[Arkatufus]

Changes

• Add new property ActiveTimers to ITimerScheduler to retrieve all currently active timer keys
• Adds ActorBase lifecycle flow unit tests in relation to IWithTimers lifecycle

Checklist

For significant changes, please ensure that the following have been completed (delete if not relevant):

• This change follows the Akka.NET API Compatibility Guidelines.
• I have reviewed my own pull request.
• Changes in public API reviewed, if any.

[Aaronontheweb]

need to make the read operation zero-allocation.

[Aaronontheweb]

I would just return an ICollection<object> or whatever here so we don't have to allocate an immutable array each time.

[Aaronontheweb]

I understand that this is probably done for the purposes of making sure we don't get rugged by the collection changing out from under us, but it's better to take that risk and leave the API un-opionated

[Arkatufus]

Done, though accessing _timers.Keys actually allocates internally inside the Dictionary, but that's unavoidable.

[Aaronontheweb]

LGTM