✅(vitest) Make tests run concurrently

[dubzzz]

Description

They used to be running in a concurrent fashion in the past. But we stopped doing it due to a recent regression on the stability of our CI with Node 23.

The regression being still noticeable even without it and because of the cost of not doing the runs in a concurrent fashion, we move back to concurrent execution.

Regression was not related to vitest, but we dropped concurrent on it too in the past.

Checklist — Don't delete this checklist and make sure you do the following before opening the PR

• The name of my PR follows gitmoji specification
• My PR references one of several related issues (if any)
  • New features or breaking changes must come with an associated Issue or Discussion
  • My PR does not add any new dependency without an associated Issue or Discussion
• My PR includes bumps details, please run yarn bump and flag the impacts properly
• My PR adds relevant tests and they would have failed without my PR (when applicable)

Advanced

• Category: ✅ Add or update tests
• Impacts: Faster CI

[changeset-bot]

⚠️ No Changeset found

Latest commit: aa65c8f

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit aa65c8f:

Sandbox	Source
@fast-check/examples	Configuration

[github-actions]

👋 A preview of the new documentation is available at: http://671fe1e4e834e838f08732c6--dubzzz-fast-check.netlify.app

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.27%. Comparing base (e0d1ced) to head (aa65c8f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5380   +/-   ##
=======================================
  Coverage   95.27%   95.27%           
=======================================
  Files         234      234           
  Lines       10478    10478           
  Branches     2795     2795           
=======================================
  Hits         9983     9983           
  Misses        495      495           

Flag	Coverage Δ
unit-tests	95.27% <ø> (ø)
unit-tests-18.x-Linux	95.27% <ø> (ø)
unit-tests-20.x-Linux	95.27% <ø> (ø)
unit-tests-22.x-Linux	95.27% <ø> (ø)
unit-tests-latest-Linux	95.27% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.