tests: fix artstation download tests.

ArtStation is apparently returning images with smaller filesizes for
/4k/ images now. It doesn't look like we can get the old images, so just
update the tests.

test/unit/danbooru_http_test.rb

@@ -152,7 +152,7 @@ class DanbooruHttpTest < ActiveSupport::TestCase
         response = Danbooru::Http.use(:unpolish_cloudflare).get(url)
 
         assert_equal(200, response.status)
-        assert_equal(720_743, response.content_length)
+        assert_equal(622_594, response.content_length)
       end
     end