my_module.ex

defmodule MyModule do
  defmacro __using__(_) do
    quote do
      @before_compile MyModule
    end
  end
  
  defmacro __before_compile__(_) do
    quote do
      setup do
        IO.puts "in the setup"
      end
    end
  end
end

test_file.exs

defmodule MyTest do
  use ExUnite.Case
  use MyModule
end