add test for lazy loading hook

test/failure_app_test.rb

@@ -337,4 +337,10 @@ class FailureTest < ActiveSupport::TestCase
       end
     end
   end
+
+  context "Lazy loading" do
+    test "loads" do
+      assert_equal Devise::FailureApp.new.lazy_loading_works?, "yes it does"
+    end
+  end
 end

test/rails_app/config/initializers/devise.rb

@@ -179,4 +179,9 @@ Devise.setup do |config|
   #   manager.failure_app = AnotherApp
   #   manager.default_strategies(scope: :user).unshift :some_external_strategy
   # end
+
+  ActiveSupport.on_load(:devise_failure_app) do
+    require "lazy_load_test_module"
+    include LazyLoadTestModule
+  end
 end

test/rails_app/lib/lazy_load_test_module.rb

@@ -0,0 +1,5 @@
+module LazyLoadTestModule
+  def lazy_loading_works?
+    "yes it does"
+  end
+end