Background

I have a small function with some branching logic and I have some tests for it. I would like to use the erlang debugger when I run a given test to make sure I am executing the correct code.

Code

Lets assume we are testing DebuggerTest.greet/1 :

defmodule DebugerTest do
  def greet(greeting) do
    if greeting == "Hola" do
      str = "#{greeting} mundo!"
      IO.puts(str)
      :spanish_greet
    else
      str = "#{greeting} world!"
      IO.puts(str)
      :normal_greet
    end
  end
end

Now, this function has some branching logic, and the code is overall horrible, but it does give us a lot of juicy executable lines that we attach a breakpoint to.
These are the tests I am running:

defmodule DebugerTestTest do
  use ExUnit.Case
  doctest DebugerTest

  test "greets the world" do
    assert DebugerTest.greet("Hello") == :normal_greet
  end

  test "greets the world in spanish" do
    assert DebugerTest.greet("Hola") == :fail_greet
  end
end

As you can see, my spanish greeting test will fail. However, I want to find that out using the erlang debugger: http://blog.plataformatec.com.br/2016/04/debugging-techniques-in-elixir-lang/

Running the tests

Given that I am only interested in running the failing test, I change my code to:

@tag :wip
 test "greets the world in spanish" do
    assert DebugerTest.greet("Hola") == :fail_greet
  end

And then I run mix test --only wip .
There, now I am running only the failing test. But I am still not using the debugger. To try to fix this I tried running the tests via an iex shell iex -S mix test --only wip but the run is automated and it ends before I can setup the debugger:

:debugger.start()
:int.ni(DebugerTest)
:int.break(DebugerTest, 3)

Question

How can I run the erlang debugger when using mix test ?