add tests

test/support/external_format_handler.ex

@@ -0,0 +1,7 @@
+defmodule FeedReader.TestSupport.ExternalFormatHandler do
+  @moduledoc false
+  @behaviour Saxy.Handler
+
+  @impl true
+  def handle_event(_event_type, _data, state), do: {:ok, state}
+end