Add JavascriptPackageTest for Action Text

This mirrors the existing `JavascriptPackageTest` tests for UJS, Action
Cable, and Active Storage.

actiontext/test/javascript_package_test.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+class JavascriptPackageTest < ActiveSupport::TestCase
+  def test_compiled_code_is_in_sync_with_source_code
+    compiled_files = %w[
+      app/assets/javascripts/actiontext.js
+    ].map do |file|
+      Pathname(file).expand_path("#{__dir__}/..")
+    end
+
+    assert_no_changes -> { compiled_files.map(&:read) } do
+      system "yarn build", exception: true
+    end
+  end
+end