Add ability to test rules e.g. with pytest

[spacemanspiff2007]

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

Identified things to do:

• 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)

[spacemanspiff2007]

@DerFetzer
Since you offered to help: I am currently struggling with the RuleRunner.
Since callbacks can be both synchronous and asynchronous I'm thinking about making the rulerunner fully async.
However that would require reworking of all tests and all docs examples and the script that runs the examples so I'm currently hesitant to go that way.
However I can't get the tests to pass - maybe you have an idea?
The Dev branch is up to date ...