Add regression test for url_helpers duplication

This adds a regression test that previously failed checking controllers
don't have multiple ancestors which include url_helpers or path_helpers.

This is checked by digging through the ancestors and seeing if they
include either the url_helpers_module or path_helpers_module and making
sure only one ancestor does that.

We usually don't test performance changes (and also would not prefer to
test implementation this closely), but because this has regressed in the
past, and I think it would be relatively easy to accidentally introduce
again, I think we should test this one.

actionpack/test/controller/route_helpers_test.rb

+# frozen_string_literal: true
+
+require "abstract_unit"
+
+class RouteHelperIntegrationTest < ActionDispatch::IntegrationTest
+  def self.routes
+    @routes ||= ActionDispatch::Routing::RouteSet.new
+  end
+
+  class FakeACBase < ActionController::Base
+    # Normally done by app initialization to ActionController::Base
+    app = RouteHelperIntegrationTest
+    extend ::AbstractController::Railties::RoutesHelpers.with(app.routes)
+  end
+
+  class ApplicationController < FakeACBase
+  end
+
+  class FooController < ApplicationController
+  end
+
+  # We define many routes in these modules after they have been included into
+  # the controllers. For boot performance, it's important that we don't
+  # duplicate these modules an maked method cache invalidation expensive.
+  # https://github.com/rails/rails/pull/37927
+  test "only includes one module with route helpers" do
+    app = self.class
+
+    url_helpers_module = app.routes.named_routes.url_helpers_module
+    path_helpers_module = app.routes.named_routes.path_helpers_module
+
+    assert_operator FooController, :<, url_helpers_module
+    assert_operator ApplicationController, :<, url_helpers_module
+    assert_not_operator FakeACBase, :<, url_helpers_module
+
+    assert_operator FooController, :<, path_helpers_module
+    assert_operator ApplicationController, :<, path_helpers_module
+    assert_not_operator FakeACBase, :<, path_helpers_module
+
+    included_modules = FooController.ancestors.grep_v(Class)
+    included_modules -= [url_helpers_module, path_helpers_module]
+
+    modules_with_routes = included_modules.select do |mod|
+      mod < url_helpers_module || mod < path_helpers_module
+    end
+
+    assert_equal 1, modules_with_routes.size
+  end
+end