We have this for tests when data comes in async from another process.

defmodule TimeHelper do
  def wait_until(fun), do: wait_until(1_000, fun)

  def wait_until(0, fun), do: fun.()

  def wait_until(timeout, fun) do
    try do
      fun.()
    rescue
      ExUnit.AssertionError ->
        :timer.sleep(10)
        wait_until(max(0, timeout - 10), fun)
    end
  end
end

import TimeHelper

test "something async", %{view: view} do
  # click link
  wait_until(fn ->
    # assert that content changes
    assert render(view) =~ "Some new data"
  end)
end