Test that the model instance is deleted in the destroy method

The generated spec for the destroy method only tests the redirect, not the
actual model deletion. To do this, we need to ensure that the model is
instantiated before performing the action.

lib/generators/koi/admin_controller/templates/controller_spec.rb.tt

@@ -124,12 +124,17 @@ RSpec.describe <%= controller_class_name %>Controller do
 
   describe "DELETE /admin/<%= plural_name %>/:id" do
     let(:action) { delete polymorphic_path([:admin, model]) }
+    let!(:model) { create(:<%= singular_name %>) }
 
     it_behaves_like "requires admin"
 
     it "renders successfully" do
       action
       expect(response).to redirect_to(polymorphic_path([:admin, <%= class_name %>]))
     end
+
+    it "deletes the <%= singular_name %>" do
+      expect { action }.to change(<%= class_name %>, :count).by(-1)
+    end
   end
 end