Fix a bug with unnecessary kwargs unpacking

[mrkaaa]

Proposed changes:

• fix a bug with unnecessary kwargs unpacking in rasa.test.test_core call from rasa.test.test (see issue Issue with kwargs unpacking in rasa.test() #4798)

Status (please check what you already did):

• made PR ready for code review
• added some tests for the functionality
• updated the documentation
• updated the changelog
• reformat files using black (please check Readme for instructions)

[CLAassistant]

All committers have signed the CLA.

[erohmensing]

Thanks for the PR, we'll give it a review as soon as possible.

[erohmensing]

@imLew does this have to do with #4458?

[imLew]

@erohmensing it's the same underlying issue (having an argument called kwargs which is not the typical catch-all-additional-keyword-args, ie **kwargs), but it is not the what that issue was referring to.

Looking at this function, I think this PR fixes the superficial issue resulting from using the name kwargs as an argument name. Right now test_core will complain about "unexpected keyword argument" if kwargs is unpacked and is not empty. But this should should addressed in more comprehensive way, either changing the name kwargs or rewritting it to use **kwargs.

[imLew]

Thanks for the fix :D, this should indeed not be unpacked.

[wochinge]

Thanks for this fix @mrkaaa Could you please rebase this to go 1.5.x and update the changelog? Thanks 🙏