Fix: Only create @babel/node IPC channel when needed

[quickgiant]

Q	A
Fixed Issues?	Fixes #13259
Patch: Bug Fix?	Yes
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

Fixes an issue where babel-node always created an IPC channel for its spawned child, regardless of whether the outer babel-node process had an IPC channel itself. This caused compatibility issues with scripts expecting to run without IPC, or expecting to only run with IPC in certain scenarios, for example the exit-hook library that assumes an IPC channel will only be present when running inside PM2. In those cases, the open IPC channel can cause the script to hang after completion unless process.disconnect() is called, which does not occur when running scripts directly through node on the command line.

To fix, the IPC channel is only created if the babel-node process itself has an IPC channel, in which case messages are expected to be passed through.

In addition, tests for this behavior have been consolidated in babel-node's fixtures, which now have options for handling IPC.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/45975/

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[JLHwung]

Thanks for changes to fixture runner.

[nicolo-ribaudo]

Thanks!