Add test for weird world locations behaviour

Added in to explain a bit of the oddity here since it could get lost in
the annals of time.

test/presenters/content_item/linkable_test.rb

@@ -41,4 +41,22 @@ def initialize
     expected_from_links = []
     assert_equal expected_from_links, item.from
   end
+
+  # World locations don't have links in the Publishing API payload
+  # This weird situation is explained here:
+  # - https://github.com/alphagov/government-frontend/pull/386
+  test "when a world location is linked to" do
+    item = DummyContentItem.new
+    item.content_item["links"]["world_locations"] = [
+      {
+        "title" => "Germany",
+        "base_path" => nil,
+      }
+    ]
+
+    expected_from_links = [
+      %{<a href="/world/germany/news">Germany</a>}
+    ]
+    assert_equal expected_from_links, item.part_of
+  end
 end