Do not load root babel.config.js in tests

[nicolo-ribaudo]

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

While working on the native ESM Babel build (wip implementation: https://github.com/nicolo-ribaudo/babel/tree/native-esm), I had to add some configFile: false to our tests (you can see them for babel-core at nicolo-ribaudo@5d15dc7) because they were accidentally loading the top-level babel.config.js file.

This didn't cause problems so far because our babel.config.js has an ignore entry for packages/*/test/fixtures, but it caused problems for the ESM tests because even if a file is ignored all the plugins/presets are still resolved and require'd.

Also, tests should be isolated so it's better to make sure that they don't accidentally load an unrelated config anyway.

cc @jridgewell If I recall correctly, AMP uses our test runner plugins. I would consider this PR a bugfix, but would it cause problems to AMP?

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/44973/

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[hzoo]

yeah I'd think we'd only want to apply the config/options from options.json? Do we need to include babelrc: false as well?

[jridgewell]

We actually handle this by loading an empty config:

1. https://github.com/ampproject/amphtml/blob/d8e414a727aede5093c6e60100dc30e7c7c488f8/babel.config.js#L36
2. https://github.com/ampproject/amphtml/blob/d8e414a727aede5093c6e60100dc30e7c7c488f8/build-system/babel-config/empty-config.js

[nicolo-ribaudo]

Oh well, good to know that this solves a real problem!