@RudManusachi
It work like charm, thank you so much.
The following is a working example.

# demo.exs
Mix.install([
  {:bunt, "~> 0.2.0"}
])

defmodule Demo do
  def hello(name \\ "John") do
    ["Hello, ", :green, :bright, name]
    |> Bunt.ANSI.format
    |> IO.puts
  end
end

Demo.hello("Smahi")

# demo_test.exs
Code.require_file("demo.exs")

ExUnit.start()

defmodule DemoTest do
  use ExUnit.Case
  import ExUnit.CaptureIO

  test "hello/1" do
   output = capture_io(fn -> Demo.hello("Jahn") end)
   assert output == "Hello, \e[32m\e[1mJahn\e[0m\n"
  end
end

One important thing to keep in mind!

When we run Code.require_file/1 it compiles (executes) the file. So if we do some side effects, external service calls etc.. those will be run each time we run tests
Need to separate out the declaration and the usage.