feat: Enable "missing act" warnings in React 18 by default

[eps1lon]

What:

Enables "missing act" warnings in React 18 for test runners that implement beforeAll and afterAll (e.g. Jest).
This can be disabled the same way auto-cleanup can be disabled.

Why:

Ensures same behavior regarding missing act warnings across as React 16 and 17

How:

Set IS_REACT_ACT_ENVIRONMENT to true once before tests start.

Checklist:

• [ ] Documentation added to the
  docs site Will wait until React docs for IS_REACT_ACT_ENVIRONMENT are available.
• Tests
• [ ] TypeScript definitions updated
• Ready to be merged
  

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[codecov]

Codecov Report

Merging #994 (3c5c261) into alpha (d8c6b4d) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             alpha      #994   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          226       233    +7     
  Branches        46        47    +1     
=========================================
+ Hits           226       233    +7     

Flag	Coverage Δ
experimental	94.42% <100.00%> (+0.17%)	⬆️
latest	90.55% <100.00%> (+0.29%)	⬆️
next	94.42% <100.00%> (+0.17%)	⬆️

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

Impacted Files	Coverage Δ
src/act-compat.js	100.00% <100.00%> (ø)
src/index.js	100.00% <100.00%> (ø)

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[eps1lon]

The warning is a pretty important part of the migration story to understand why certain tests might start failing or at least excibit different behavior. At least with React 18 users are able to disable the "missing act" warning if they truly want to so this change should be fairly safe.

The only issue might if people reset their modules in beforeEach so we may want to switch to beforeEach instead of beforeAll.

[github-actions]

🎉 This PR is included in version 13.0.0-alpha.5 🎉

The release is available on:

• npm package (@alpha dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[IanVS]

For anyone else finding this and wondering what the same way auto-cleanup can be disabled is, the docs are at: https://testing-library.com/docs/react-testing-library/setup#skipping-auto-cleanup