Fix an activity test related to Notifier mock

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

test/graphql/resolvers/activity_test.exs

@@ -13,6 +13,12 @@ defmodule Mobilizon.GraphQL.Resolvers.ActivityTest do
 
   setup %{conn: conn} do
     group = insert(:group)
+
+    Mobilizon.Config.put([Mobilizon.Service.Notifier, :notifiers], [
+      Mobilizon.Service.Notifier.Email,
+      Mobilizon.Service.Notifier.Push
+    ])
+
     {:ok, conn: conn, group: group}
   end