test: add test for server

test/todo/server_test.exs

@@ -0,0 +1,14 @@
+defmodule Todo.TestServer do
+  use ExUnit.Case
+  alias Todo.Server
+
+  test "simple usage" do
+    {:ok, pid} = Server.start()
+    assert [] = Server.entries(pid, ~D[2024-08-04])
+
+    Server.add(pid, %{date: ~D[2024-08-04], title: "Write some Elixir"})
+
+    assert [%{date: ~D[2024-08-04], title: "Write some Elixir"}] =
+             Server.entries(pid, ~D[2024-08-04])
+  end
+end