00-before.ex

events = GenEvent.stream(context[:event_manager], timeout: 20)
spawn(fn() ->
  :timer.sleep(10)
  :ok = :gen_tcp.send(context[:client], "message:hello:wor:ld")
end)
assert(Enum.fetch!(events, 0) == {"hello", "wor:ld"})

01-after.ex

defmodule Handler do
  use GenEvent
  def handle_event(event, pid), do: (send(pid, event); {:ok, pid})
end
GenEvent.add_handler(context[:event_manager], Handler, self)
:ok = :gen_tcp.send(context[:client], "message:hello:wor:ld")
receive(
  do: ({"hello", "wor:ld"} -> :ok),
  after: (10 -> assert(false, "timeout"))
)