test(growth): add dep to write in-memory excel data

This will be used to mock data from WHO site.

mix.exs

@@ -39,6 +39,7 @@ defmodule Wabanex.MixProject do
       {:dialyxir, "~> 1.4.0", only: [:dev, :test], runtime: false},
       {:dns_cluster, "~> 0.1.1"},
       {:ecto_sql, "~> 3.12.0"},
+      {:elixlsx, "~> 0.6.0", only: :test},
       {:ex_doc, "~> 0.34.0", only: :dev, runtime: false},
       {:gettext, "~> 0.26.0"},
       {:jason, "~> 1.4.0"},