Add Rack::Lint to ActionDispatch::MiddlewareStack tests

To ensure Rails is and remains compliant with [the Rack 3 spec](6d16306192/UPGRADE-GUIDE.md) we can add `Rack::Lint` to the Rails middleware tests.

This adds additional test coverage to `ActionDispatch::MiddlewareStack` to validate that its input and output follow the Rack SPEC.

In this case, no changes are required, and the additional test
will ensure this middleware remains compliant with the Rack SPEC.

actionpack/test/dispatch/middleware_stack_test.rb

@@ -200,10 +200,14 @@ def test_delete_works
     end
 
     ActiveSupport::Notifications.subscribed(subscriber, "process_middleware.action_dispatch") do
-      app = @stack.build(proc { |env| [200, {}, []] })
+      app = Rack::Lint.new(
+        @stack.build(Rack::Lint.new(proc { |env| [200, {}, []] }))
+      )
 
-      env = {}
-      app.call(env)
+      env = Rack::MockRequest.env_for("", {})
+      assert_nothing_raised do
+        app.call(env)
+      end
     end
 
     assert_equal 2, events.count