Allow testMatch for jest config

[favna]

Resolves #8637.

Tested locally with these steps:

1. cd packages/react-scripts
2. yarn link
3. In another project: yarn link "react-scripts"
4. Running SKIP_PREFLIGHT_CHECKS=true yarn test
5. Verifying that nothing changed from before symlinking modules
   
6. Add testMatch key to package.json Jest config
7. Running SKIP_PREFLIGHT_CHECKS=true yarn test again
8. Verifying that only filed matching specified pattern are being included for testing
   

For further reasoning as to why this option should be added please refer to #8637.

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[favna]

Failing CI appears unrelated to this PR, as a matter of fact, at time of writing CI is also failing on master....

[mrmckeb]

I see no reason why this would affect the CI either, sorry you hit that issue.

@ianschmitz I think we can merge this one?

[ianschmitz]

Bah this error in CI is getting old...