Correctly check whether Action Cable is enabled in AuthenticationGenerator

`AuthenticationGenerator` doesn't handle `skip_action_cable` option
so far(`AppGenerator` only handles it).
https://github.com/rails/rails/blob/e6429269fd5a8a7d728557f4e7d7f82c0ad1478c/railties/lib/rails/generators/rails/authentication/authentication_generator.rb#L6-L8

So the current check doesn't work correctly.

Instead of depending on that option, this PR changes to check whether
the engine is enabled or not. We already do the same thing.
https://github.com/rails/rails/blob/e6429269fd5a8a7d728557f4e7d7f82c0ad1478c/railties/lib/rails/commands/app/update_command.rb#L71

Author: y-yagi

railties/lib/rails/generators/rails/authentication/authentication_generator.rb

@@ -19,7 +19,7 @@ def create_authentication_files
         template "app/controllers/concerns/authentication.rb"
         template "app/controllers/passwords_controller.rb"
 
-        template "app/channels/application_cable/connection.rb" unless options.skip_action_cable?
+        template "app/channels/application_cable/connection.rb" if defined?(ActionCable::Engine)
 
         template "app/mailers/passwords_mailer.rb"
 

railties/test/generators/authentication_generator_test.rb

@@ -108,11 +108,14 @@ def test_model_test_is_skipped_if_test_framework_is_given
   end
 
   def test_connection_class_skipped_without_action_cable
-    generator([destination_root], skip_action_cable: true)
-
+    old_value = ActionCable.const_get(:Engine)
+    ActionCable.send(:remove_const, :Engine)
+    generator([destination_root])
     run_generator_instance
 
     assert_no_file "app/channels/application_cable/connection.rb"
+  ensure
+    ActionCable.const_set(:Engine, old_value)
   end
 
   private