Add actionable message for "React.act is not a function" error

[andrzejressel]

I've added error message with instruction how to fix infamous React.act is not a function error

What:

I myself encountered this error and seeing number of issues in Github and StackOverflow - it seems to be pretty common issue. If NODE_ENV is set to production the error will tell user to change this env var to test.

Why:

To make initial experience simpler for new JS devs like me.

How:

By making the error return action that will probably fix the issue.

Checklist:

• Documentation added to the
  docs site N/A
• Tests (can it be tested since specific env var is required?)
• TypeScript definitions updated N/A
• Ready to be merged
  

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Sandbox	Source
react-testing-library-examples	Configuration

[andrzejressel]

To be honest - I don't know how to fix the CI - I understand that the branch is not tested, but I have no idea how to mock these env vars - especially since I believe they must be set during build time