fix(unmount): Flush useEffect cleanup functions syncronously

[eps1lon]

What:

Flush cleanup functions from useEffect synchronously on unmount.

Why:

react@next no longer flushes cleanup functions from useEffect synchronously.
Verified in https://travis-ci.org/github/testing-library/react-testing-library/builds/708314737

How:

Wrap unmountComponentAtNode in act.

Checklist:

• [ ] Documentation added to the
  docs site
• Tests
• [ ] Typescript definitions updated
• Ready to be merged
  

I'm not touching cleanup and flush-microtasks for now. We can do that in a follow-up PR that we test extensively against existing repos.

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[codecov]

Codecov Report

Merging #746 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #746   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          100       101    +1     
  Branches        16        16           
=========================================
+ Hits           100       101    +1     

Impacted Files	Coverage Δ
src/pure.js	100.00% <100.00%> (ø)

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[marcosvega91]

just a question, this should be done also here?

react-testing-library/src/pure.js

Line 109 in 240900c

ReactDOM.unmountComponentAtNode(container)

EDIT
ok I read your note at the end of the PR

[kentcdodds]

Makes sense. Thanks 👍

[kentcdodds]

🎉 This PR is included in version 10.4.7 🎉

The release is available on:

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

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