Unlike the webpack config, webpacker.yml settings are determined by the current RAILS_ENV.

All this to say: your webpack test config, i.e. config/webpack/test.js, is essentially useless unless your application: a. uses a Node.js test runner for JavaScript unit test AND configure it to use your webpack config a. overrides the defaults so that the test config is loaded in your Rails tests (just be sure to change Babel behavior)