Use babel-register script when running babel smoke tests (#442)

scripts/test-babel-register.js

@@ -0,0 +1,10 @@
+"use strict";
+
+const register = require("babel-register").default;
+
+register({
+  extensions: [".js"],
+  // Only js files in the test folder but not in the subfolder fixtures.
+  only: [/packages\/.+\/test\/(?!fixtures\/).+\.js$/],
+  compact: true,
+});