add tests

test/support/transport_client.ex

@@ -6,6 +6,14 @@ defmodule FeedReader.TestSupport.TransportClient do
   @impl true
   def request(url, opts) do
     send(opts[:test_pid], {:transport_request, url})
-    {:ok, %{status: 200, headers: [], final_url: url, body: ["test"]}}
+
+    case Keyword.get(opts, :request_result) do
+      nil ->
+        {:ok,
+         Keyword.get(opts, :response, %{status: 200, headers: [], final_url: url, body: ["test"]})}
+
+      result ->
+        result
+    end
   end
 end