Avoid using robohash for factory generated article cover images (#17024)

* Avoid using robohash for article cover images

If their service slows down, loading the cover image times out in
system tests.

Use a locally served image file instead.

* Specify expected image format for view object test

We were relying on the internals of Faker to return a robohash.org url
and checking that we included that in the cloudinary url.

Rather than relying on the default behavior, explicitly pass a
robohash url in the main image for this spec.

spec/factories/articles.rb

@@ -23,7 +23,11 @@ FactoryBot.define do
     co_author_ids { [] }
     association :user, factory: :user, strategy: :create
     description { Faker::Hipster.paragraph(sentence_count: 1)[0..100] }
-    main_image    { with_main_image ? Faker::Avatar.image : nil }
+    main_image    do
+      if with_main_image
+        URL.url(ActionController::Base.helpers.asset_path("#{rand(1..40)}.png"))
+      end
+    end
     experience_level_rating { rand(4..6) }
     body_markdown do
       <<~HEREDOC

spec/view_objects/cloud_cover_url_spec.rb

@@ -1,7 +1,7 @@
 require "rails_helper"
 
 RSpec.describe CloudCoverUrl, type: :view_object, cloudinary: true do
-  let(:article) { create(:article) }
+  let(:article) { create(:article, main_image: "https://robohash.org/articlefactory.png") }
   let(:cloudinary_prefix) { "https://res.cloudinary.com/#{Cloudinary.config.cloud_name}/image/fetch/" }
 
   it "returns proper url" do