add tests

test/support/transport_client.ex

@@ -0,0 +1,11 @@
+defmodule FeedReader.TestSupport.TransportClient do
+  @moduledoc false
+
+  @behaviour FeedReader.Transport
+
+  @impl true
+  def request(url, opts) do
+    send(opts[:test_pid], {:transport_request, url})
+    {:ok, %{status: 200, headers: [], final_url: url, body: ["test"]}}
+  end
+end