tests: fix more spurious test failures in CI.

* Skip Nijie tests because they fail a lot due to Nijie rate limiting us.
* Skip ArtStation downloads tests because they sometimes return different file sizes.
* Fix random duplicate favgroup errors because favgroup names weren't random enough.

test/factories/favorite_group.rb

@@ -1,6 +1,6 @@
 FactoryBot.define do
   factory :favorite_group do
     creator
-    name { FFaker::Lorem.word }
+    name { SecureRandom.uuid }
   end
 end

test/unit/danbooru_http_test.rb

@@ -157,6 +157,8 @@ class DanbooruHttpTest < ActiveSupport::TestCase
 
     context "unpolish cloudflare feature" do
       should "return the original image for polished images" do
+        skip if ENV["CI"].present?
+
         url = "https://cdnb.artstation.com/p/assets/images/images/025/273/307/4k/atey-ghailan-a-sage-keyart-s-ch-04-outlined-1.jpg?1585246642"
         response = Danbooru::Http.use(:unpolish_cloudflare).get(url)
 

test/unit/downloads/art_station_test.rb

@@ -10,6 +10,8 @@ module Downloads
 
     context "for an image where an original does not exist" do
       should "not try to download the original" do
+        skip if ENV["CI"].present?
+
         assert_downloaded(452_795, "https://cdna.artstation.com/p/assets/images/images/004/730/278/large/mendel-oh-dragonll.jpg")
       end
     end

test/unit/sources/nijie_test.rb

@@ -4,8 +4,7 @@ module Sources
   class NijieTest < ActiveSupport::TestCase
     setup do
       skip "Nijie credentials not configured" unless Sources::Strategies::Nijie.enabled?
-      # Add a random delay to work around test failures due to rate limiting by Nijie.
-      sleep (3..5).to_a.sample
+      skip if ENV["CI"].present?
     end
 
     context "downloading a 'http://nijie.info/view.php?id=:id' url" do