Use real filesystem for unit testing

[bluwy]

Changes

Use the real filesystem for unit testing instead of mocking fs with memfs. This unblocks #12165. Benefits:

1. We can view the actual files that are being run.
2. We rely on the real filesystem, ensuring path handling or any OS specific behaviours are tested.
3. We no longer have to change our implementation to allows passing in a fs param.

---

The PR uses fs-fixture (with a patch that I'll comment below) that allows us to create files and directories as fixtures to test on. It's merging to main instead of next so that if anyone adds a new unit test in the future in main, they can use this pattern. Otherwise we have to manually migrate whenever merging main to next.

This means after the PR is merged (I hope!), we can start removing our fs abstraction that was introduced only for tests, which should simply things more.

Testing

Existing test should pass, especially unit tests

Docs

n/a

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[bluwy]

I need to fix the test fail, but thanks for the very quick review 😅

[ematipico]

Happy to merge!

[bluwy]

Thanks for the reviews. I had trouble making restart.test.js working as there's issues with chokidar detecting changes. I resorted to the mock event emitting trick like before to make it happy for now, but it's something I want to take a better look in the future. I'll merge this for now so we can unblock the other PRs.