Example of GenServer with Tests

counter.ex

defmodule Counter do
  use GenServer

  def start_link(_) do
    GenServer.start_link(__MODULE__, 0, name: __MODULE__)
  end

  @impl true
  def init(initial), do: {:ok, initial}

  @impl true
  def handle_call(:count, _from, state), do: {:reply, state, state}

  def handle_call(:increment, _from, state), do: {:reply, state + 1, state + 1}

  def handle_call(:decrement, _from, state), do: {:reply, state - 1, state - 1}

  def count, do: GenServer.call(__MODULE__, :count)
  def increment, do: GenServer.call(__MODULE__, :increment)
  def decrement, do: GenServer.call(__MODULE__, :decrement)
end

counter_test.ex

defmodule CounterTest do
  use ExUnit.Case, async: true

  setup do
    start_supervised!(Counter)
    %{}
  end

  test "if Counter loads with initial state of 0" do
    assert Counter.count() === 0
  end

  test "if Counter.count/0 return the current state" do
    assert Counter.count() === 0
    assert Counter.increment() === 1
    assert Counter.count() === 1
  end

  test "if Counter.increment/0 increases state to +1 and return the current state" do
    assert Counter.count() === 0
    assert Counter.increment() === 1
    assert Counter.count() === 1
    assert Counter.increment() === 2
    assert Counter.count() === 2
  end

  test "if Counter.decrement/0 decrements state to -1 and return the current state" do
    assert Counter.count() === 0
    assert Counter.increment() === 1
    assert Counter.count() === 1
    assert Counter.decrement() === 0
    assert Counter.count() === 0
    assert Counter.decrement() === -1
    assert Counter.count() === -1
  end
end