my_sum.exs

defmodule MySum do
  def sum(x, y), do: x + y
end

my_sum_test.exs

ExUnit.start()

defmodule MySumTest do
  use ExUnit.Case, async: true

  Code.require_file("./my_sum.exs")

  test "it gives 3 when inputs are 1 and 2" do
    assert(MySum.sum(1, 2) == 3)
  end
end

You can run this example by putting both files in the same folder and then on the command line, whilst in that folder, running elixir my_sum_test.exs.