DOP-4701: Preserve stdout in ShellCommandExecutor

[i80and]

Stories/Links:

DOP-4701

Notes

The actual fix is the one line change in commandExecutor.ts.

The annoying part was making the test work:

• The existing test incorrectly mocked the child_process.exec() interface.

• According to the node docs, the exception raised by util.promisify(child_process.exec()) will always have a stdout property. But just in case the exception get raised by something else in the try block, it'll fall back to returning '' as in the current behavior.

• util.promisify(child_process.exec) has a custom promisifier (child_process.exec[util.promisify.custom]) that considerably changes the interface, but that jest loses when you call jest.mock('child_process'), and because of jest magic tomfoolery, I couldn't find any tidy way to restore that custom hook, which is needed to actually make the interface behave correctly.

  So I thought, you know what? Why mock? What value do we get by mocking child_process.exec besides introducing surface areas for bugs?

  So this PR just executes commandExecutor.execute(["echo 'stdout output'", "echo 'stderr output' >&2", 'exit 1']), which gives us everything we need to make sure ShellCommandExecutor actually works, using only shell builtins that we already require.

README updates

• • This PR introduces changes that should be reflected in the README, and I have made those updates.
• • This PR does not introduce changes that should be reflected in the README

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[mmeigs]

Thank you for the explanations!!