Monkey-patch AppName#valid_const? on older Rails (< 7.1)

My hunch is that this was fixed in Rails 7.1 by rails/rails#46074, but I'm less concerned here because this test suite uses a Dummy app for all of the tests.

This is a problem I've been working on in rails/rails#50427, to remove the dummy applications and replace them with a generated app like we've done here with the installer test.

I'm happy to investigate replacing the dummy app here afterwards.

test/app_helper.rb

@@ -2,6 +2,15 @@
 require "rails/test_help"
 require "fileutils"
 
+if Gem::Version.new(Rails.version) < Gem::Version.new("7.1")
+  module Rails::Generators::AppName
+    private
+      def valid_const?
+        true
+      end
+  end
+end
+
 module RailsAppHelpers
   private
   def create_new_rails_app(app_dir, options=[])