[5.x] Use node instead of webpack config for jest

Backports PR elastic#12974

src/jest/babelTransform.js

@@ -1,6 +1,6 @@
 const babelJest = require('babel-jest');
 const options = require('../optimize/babel/options');
 
-const babelOptions = options.webpack;
+const babelOptions = options.node;
 
 module.exports = babelJest.createTransformer(babelOptions);