test: add mock MRF module for describe() testing

test/support/mrf_module_mock.ex

@@ -0,0 +1,13 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2018 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule MRFModuleMock do
+  @behaviour Pleroma.Web.ActivityPub.MRF
+
+  @impl true
+  def filter(message), do: {:ok, message}
+
+  @impl true
+  def describe(), do: %{"mrf_module_mock" => "some config data"}
+end