Update JavascriptPackageTest for Active Storage

Prior to this commit, if `app/javascript/activestorage/index.js`
contained a syntax error, `JavascriptPackageTest` would still pass
because `system "yarn build"` would simply return `false` and the
compiled output would not change.  This commit adds `exception: true` to
the `system` call so that an error will be raised if `yarn build` fails.

Also, since 6c96e1c, Active Storage
compiles an additional `app/assets/javascripts/activestorage.esm.js`
file.  This commit adds an assertion for that file as well.

Author: jonathanhefner

activestorage/test/javascript_package_test.rb

@@ -4,10 +4,15 @@
 
 class JavascriptPackageTest < ActiveSupport::TestCase
   def test_compiled_code_is_in_sync_with_source_code
-    compiled_file = File.expand_path("../app/assets/javascripts/activestorage.js", __dir__)
+    compiled_files = %w[
+      app/assets/javascripts/activestorage.js
+      app/assets/javascripts/activestorage.esm.js
+    ].map do |file|
+      Pathname(file).expand_path("#{__dir__}/..")
+    end
 
-    assert_no_changes -> { File.read(compiled_file) } do
-      system "yarn build"
+    assert_no_changes -> { compiled_files.map(&:read) } do
+      system "yarn build", exception: true
     end
   end
 end