Use if instead of skip to skip tests

This way the tests won't appear as "pending" when running the test
suite, and so we get rid of a lot of noise in the test results. There
doesn't seem to be a way to call `skip` without the test being marked as
"pending".

Note that in the globalizable tests we need to build a factory before
deciding whether an atribute is required or not (particularly for the
milestone factory, since milestone attributes are required depending on
the presence of other attributes). This isn't possible before we're
inside the test, so we can't add an `if:` condition to the test. So
we're adding the condition inside the test instead. A minor
inconvenience of this method is the test still runs even when the
condition is `false`.

spec/shared/system/nested_imageable.rb

@@ -128,8 +128,8 @@ shared_examples "nested imageable" do |imageable_factory_name, path, imageable_p
       end
     end
 
-    scenario "Render image preview after sending the form with validation errors" do
-      skip "Question answers behave differently" if imageable.is_a?(Poll::Question::Answer)
+    scenario "Render image preview after sending the form with validation errors",
+             unless: imageable_factory_name == "poll_question_answer" do
       do_login_for user, management: management
       visit send(path, arguments)
 
@@ -154,17 +154,14 @@ shared_examples "nested imageable" do |imageable_factory_name, path, imageable_p
       expect(page).not_to have_selector("#nested-image .image")
     end
 
-    scenario "Should show successful notice when resource filled correctly without any nested images" do
-      if has_many_images
-        skip "no need to test, there are no attributes for the parent resource"
-      else
-        do_login_for user, management: management
-        visit send(path, arguments)
+    scenario "Should show successful notice when resource filled correctly without any nested images",
+             unless: has_many_images do
+      do_login_for user, management: management
+      visit send(path, arguments)
 
-        send(fill_resource_method_name) if fill_resource_method_name
-        click_on submit_button
-        expect(page).to have_content imageable_success_notice
-      end
+      send(fill_resource_method_name) if fill_resource_method_name
+      click_on submit_button
+      expect(page).to have_content imageable_success_notice
     end
 
     scenario "Should show successful notice when resource filled correctly and after valid file uploads" do