Tests: Add a helper method to reduce sleeping times in test.

This will 'time travel', i.e. change the inserted_at and update_at
fields of the object in question. This is used to backdate things
were we used sleeping before to ensure time differences.

test/support/helpers.ex

@@ -55,6 +55,14 @@ defmodule Pleroma.Tests.Helpers do
           clear_config: 2
         ]
 
+      def time_travel(entity, seconds) do
+        new_time = NaiveDateTime.add(entity.inserted_at, seconds)
+
+        entity
+        |> Ecto.Changeset.change(%{inserted_at: new_time, updated_at: new_time})
+        |> Pleroma.Repo.update()
+      end
+
       def to_datetime(%NaiveDateTime{} = naive_datetime) do
         naive_datetime
         |> DateTime.from_naive!("Etc/UTC")