Fix error messages for question answer images

Since we were creating a new answer in the form, we weren't getting the
errors associated to the answer the administrator was trying to create,
and so we were skipping the test.

Using the answer which contains the information about validation errors
fixes the issue and so we don't have to skip the tests.

app/views/admin/poll/questions/answers/images/new.html.erb

@@ -1,5 +1,5 @@
 <div class="poll-question-form">
-  <%= form_for(Poll::Question::Answer.new,
+  <%= form_for(@answer,
                url: admin_answer_images_path(@answer),
                method: :post) do |f| %>
     <%= render "shared/errors", resource: @answer %>

spec/shared/system/nested_imageable.rb

@@ -119,12 +119,8 @@ shared_examples "nested imageable" do |imageable_factory_name, path, imageable_p
       click_link "Add image"
       click_on submit_button
 
-      if has_many_images
-        # Pending. Review soon and test
-      else
-        within "#nested-image .image" do
-          expect(page).to have_content("can't be blank", count: 2)
-        end
+      within "#nested-image .image" do
+        expect(page).to have_content("can't be blank", count: 2)
       end
     end