add tests

test/support/external_format.ex

@@ -0,0 +1,25 @@
+defmodule FeedReader.TestSupport.ExternalFormat do
+  @moduledoc false
+  @behaviour FeedReader.Format
+
+  alias FeedReader.{Feed, Item}
+
+  @impl true
+  def id, do: :external_test
+
+  @impl true
+  def matches_prefix?("<external-feed" <> _rest), do: true
+  def matches_prefix?(_prefix), do: false
+
+  @impl true
+  def handler, do: FeedReader.TestSupport.ExternalFormatHandler
+
+  @impl true
+  def new_parser_state(event_sink, opts), do: %{event_sink: event_sink, opts: opts}
+
+  @impl true
+  def normalize_feed(_state, _opts), do: %Feed{title: "External", type: :rss}
+
+  @impl true
+  def normalize_item(%{title: title}, _feed, _opts), do: {:ok, %Item{title: title}}
+end