Add test for compiled @rails/ujs

actionview/test/javascript_package_test.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+class JavascriptPackageTest < ActiveSupport::TestCase
+  def test_compiled_code_is_in_sync_with_source_code
+    assert_no_changes -> {
+      %w[
+        app/assets/javascripts/rails-ujs.js
+        app/assets/javascripts/rails-ujs.esm.js
+      ].map { |compiled_file|
+        File.read(File.expand_path("../#{compiled_file}", __dir__))
+      }
+    } do
+      system "yarn build"
+    end
+  end
+end