Add ability to test rules e.g. with pytest

[spacemanspiff2007]

trafficstars

It would be nice if the rules could be easily tested e.g. with pytest.

Identified things to do:

• [x] Create a way to import rule classes in other files without executing them
• [ ] Mock items
• [ ] Time Travel
• [ ] Bind global states (event bus, item registry) to runtime object
• [ ] Mock runtime object
• [ ] Time travel for item constant watcher

Existing testing implementations DerFetzner nobbi1991

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@DerFetzer I've opened this issue so we can track progress here instead of your PR. The rework of the scheduler was lots of work, way more than I anticipated but I have not forgotten about the issue. I'm gradually working on it, but unfortunately time is limited at the moment (as always).

The most difficult thing is that a new rule runner will have to be run asynchronously and that makes things much more difficult ...

Creating rules in an importable way already works:

from HABApp.rule import create_rule,

create_rule(MyRuleClass, arg1)