Expose actor restarting fixtures publicly

[llucax]

The fixtures to avoid infinite restarts of actors is generally useful for anyone authoring actors, so it would be useful to expose these fixtures publicly in some test package (probably also add the dependencies for this as optional test dependencies, so they are not pulled in prod environment).

https://github.com/frequenz-floss/frequenz-sdk-python/blob/v1.x.x/tests/conftest.py

Originally posted by @llucax in frequenz-floss/frequenz-dispatch-python#89 (comment)