Use ES modules and dynamic import for Webpack latest builds (#16620)

test/webpack.config.js

@@ -1,8 +1,13 @@
 import webpack from "../build-scripts/webpack.cjs";
 
-export default webpack.createAppConfig({
+const config = webpack.createAppConfig({
   isProdBuild: false,
   latestBuild: true,
   isStatsBuild: false,
   isTestBuild: true,
 });
+
+// instant-mocha forces a CJS library, so cannot output ESM
+config.output.module = false;
+
+export default config;