[code-infra] Support parallel tests

[oliviertassinari]

Summary

Our mocha tests run serially which is inefficient and slow. For example in https://github.com/mui/mui-x, https://app.circleci.com/pipelines/github/mui/mui-x/60861/workflows/e2ea2b79-d335-4e79-874b-656570f42f38/jobs/347360/steps tests are way too slow >14 minutes, and only 1/2 of the CPUs available.

We should run these tests in parallel. Mocha doesn't support it mochajs/mocha#4958.

Examples

• Vitest: https://vitest.dev/config/#pool

[Janpot]

We're doing a Pilot in X to assess the extent of a migration to vitest. We're sharding in Toolpad, easy to set up, works like a breeze.

cc @JCQuintas

[oliviertassinari]

I wonder if it wouldn't be easier to first speed up the tests. Maybe we do a lot of repetitive work between each test that we shouldn't. I recall that with Sebastian, when we moved from shallow tests to react testing library, we found a couple of perf issues in jsdom and testing-library selectors.

[JCQuintas]

Hum, for X we might be better off just moving to vitest first, then optimising.

My current approach is to try to do only slim modifications of @mui/internal-test-utils so that it works for both vitest(jsdom&browser) and mocha/karma.

Which should allow us to move into vitest package by package.

You can follow the prs here:
mui/mui-x#14508
mui/material-ui#43625