[test] Revert to using fireEvent instead of userEvent

[LukasTy]

Revert changes introduced in #14142 in an effort to avoid leaking configuration and flakily failing tests.

There were no flaky test_unit failures on this branch. 👌

[mui-bot]

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[cherniavskii]

Why revert these changes though?

[LukasTy]

Why revert these changes though?

@cherniavskii because the amount of flaky Picker test failures has been too intense lately.
It seems that they started after introducing more async tests to Pickers.

I have tried moving most of the Pickers tests to async in #14583 and it caused way more flaky failures. 🙈

It seems that we can not reliably mix async and sync tests while we are on mocha because it has trouble isolating test contexts. 🤷

[LukasTy]

However, for some reason, I haven't seen test:unit fail flakily in the past 10 master merges.

How has it been in everyone's branches? 🤔

[LukasTy]

I've converted the PR to a draft until we see a new flaky test:unit pipeline failure.

[LukasTy]

Here we go again... 🙈
https://app.circleci.com/pipelines/github/mui/mui-x/68873/workflows/1691361d-2eee-491a-b2e5-a84c73131ae5/jobs/392537
So, the flakiness is not gone, it's just varied. 😆

[cherniavskii]

@LukasTy Do you think we can have the material-ui-v6 workflow passing in this PR?

[LukasTy]

Do you think we can have the material-ui-v6 workflow passing in this PR?

This (7a66b0e) seemed to do the trick.
At least locally. 👌

[LukasTy]

I have added the @mui/utils bump to make it more correct. 🤔
I feel like after #14867 it makes sense. 🤷

[LukasTy]

The PR is ready for review.
Hopefully, it stabilizes the test_unit CI step. 🤞

[cherniavskii]

Looks good! 💙

[cherniavskii]

Some tests are still flaky on the master branch: b3700ae
Not sure it's worth so spend much time on trying to fix them at this point.