Swap out genserver implementation per test

GenServerSwapOutTest.ex

defmodule MyService do
  def foo() do
    GenServer.call(:my_service, :foo)
  end
end

defmodule MyServiceFake do
  use GenServer
  def handle_call(:foo, _from, state) do
    {:reply, :fake, state}
  end

  def start_link do
    GenServer.start_link(__MODULE__, nil, name: :my_service)
  end
end

defmodule MyService4Real do
  use GenServer
  def handle_call(:foo, _from, state) do
    {:reply, :real, state}
  end

  def start_link do
    GenServer.start_link(__MODULE__, nil, name: :my_service)
  end
end

defmodule GenServerSwapOutTest do
  use ExUnit.Case

  setup(%{service: module}) do
    module.start_link()
    :ok
  end

  @tag service: MyServiceFake
  test "test fake" do
    assert MyService.foo() == :fake
  end

  @tag service: MyService4Real
  test "test real" do
    assert MyService.foo() == :real
  end
end