Move custom page 404 test to controller specs

So it's similar to the other custom page 404 tests.

Note we're explicitely marking the page as a draft so it's more obvious
what's going on in the test.

spec/controllers/pages_controller_spec.rb

@@ -45,5 +45,13 @@ describe PagesController do
       get :show, params: { id: "nonExistentJavaScript.js" }
       expect(response).to be_not_found
     end
+
+    it "returns a 404 message for draft pages" do
+      create(:site_customization_page, slug: "other-slug", status: "draft")
+
+      get :show, params: { id: "other-slug" }
+
+      expect(response).to be_not_found
+    end
   end
 end