feat(jest-core): Add newline after Json output

[jessevanassen]

Summary

This pull request adds newline characters at the end of Json objects produced with jest --json.

This makes it trivial to separate multiple test runs when running jest --json --watch. With the current behavior, all result objects will be concatenated without separators, requiring a streaming or custom parser to separate the test runs.

It also makes the output conform to the Unix file specification, which dicates that lines in a text file should end with an \n.

Fixes #13816

Test plan

All existing tests still pass.

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[SimenB]

can you add an integration test verifying the newline is present?

[jessevanassen]

can you add an integration test verifying the newline is present?

On it! 👍

[jessevanassen]

I've amended the existing tests for the Json reporter to check for the final \n, for both the variant that writes to a file and to stdout.

I also needed to add an option to runJest to preserve the final newline in stdout / stderr. The tests use Execa to spawn Jest, which by default strips the final newline. Not great if you explicitly want to test if the final new line is present 😄.
I made it configurable so that the trailing new line can be preserved, but by default will be stripped to keep the original behavior.

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Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!

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Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!

[SimenB]

thanks!

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