add test stack

lib/stack.ex

@@ -0,0 +1,19 @@
+defmodule MyStack do
+  use GenServer
+
+  def start_link(args) do
+    GenServer.start_link(__MODULE__, nil, name: args[:name])
+  end
+
+  def init(_) do
+    {:ok, []}
+  end
+
+  def handle_info({:push, el}, state) do
+    {:noreply, [el] ++ state}
+  end
+
+  def handle_call(:pop, _from, [head | tail]) do
+    {:reply, head, tail}
+  end
+end