[Doc] Fix examples of ra-test with fake dataProviders

[DjebbZ]

You helped me in #6597, so I figured I could make the docs better for future me and everyone.

I haven't tested the code to see if it works. Also, a few notes:

• contrarily to some test I've seen in the react-admin code base, I didn't need the await act(async () => await new Promise(r => setTimeout(r))); dance. It seems that simply awaiting was enough to follow properly the event loop.
• I've simplified the jest.fn().resolve since the examples weren't using the mocked function for some assertions
• I've replace queryBy with findBy which is the asynchronous equivalent. In my own code, it's required or the rendered content is the inital one before the dataProvider was called.

Feel free to adjust the code before merging, there's no guarantee it's correct in the context of the examples.

[DjebbZ]

Furthermore, .resolve() doesn't exist on jest.fn().

[fzaninotto]

Excellent, thanks!

[DjebbZ]

You're welcome!