Do not require `webpacker` in Active Job integration tests

Because `skip-javascript` option is specified for the dummy application
used in the integration tests.
https://github.com/rails/rails/blob/bdc96802a7a4a8c6d183272e997430e8b9006498/activejob/test/support/integration/helper.rb#L14

activejob/Rakefile

@@ -28,6 +28,7 @@ namespace :test do
 
   task "env:integration" do
     ENV["AJ_INTEGRATION_TESTS"] = "1"
+    ENV["SKIP_REQUIRE_WEBPACKER"] = "true"
   end
 
   ACTIVEJOB_ADAPTERS.each do |adapter|