Avoid model inheritance in skip validation test

One of these tests was failing sometimes on Github Actions. It looked
like the line `custom_banner.save!` was using the validations from the
Banner class sometimes, even if the callbacks had correctly been
removed in the DummyBanner class.

Se we're inheriting from ApplicationRecord instead of inheriting from
Banner. Since I couldn't reproduce the issue locally after running the
test hundreds of times and with the same seed and tests that were
running on Github Actions, there's a change this won't work. I've tested
a few times on Github Actions and it seems to be working, but we'll have
to keep an eye on it.

spec/models/skip_validation_spec.rb

@@ -49,19 +49,21 @@ describe SkipValidation do
 
   describe ".skip_translation_validation" do
     before do
-      dummy_banner = Class.new(Banner) do
-        def self.translation_class
-          @translation_class ||= Class.new(Banner::Translation) { clear_validators! }
+      dummy_banner = Class.new(ApplicationRecord) do
+        def self.name
+          "DummyBanner"
         end
-        reflect_on_association(:translations).options[:class_name] = "DummyBanner::Translation"
+        self.table_name = "banners"
+
+        translates :title, touch: true
+        translates :description, touch: true
+        include Globalizable
 
-        clear_validators!
         validates_translation :title, presence: true
         validates_translation :description, presence: true
       end
 
       stub_const("DummyBanner", dummy_banner)
-      stub_const("DummyBanner::Translation", dummy_banner.translation_class)
     end
 
     it "removes the validation from the translatable attribute" do