How to run ExUnit tests with erlang debugger?

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:

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 ?

Try iex -S mix test --trace --only wip.

Doesnt work:

iex -S mix test --trace
Erlang/OTP 23 [erts-11.2] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [hipe]

Hello world!
Hola mundo!

DebugerTestTest [test/debuger_test_test.exs]
  * test greets the world (0.03ms) [L#5]
  * test greets the world in spanish (0.02ms) [L#9]


Finished in 0.02 seconds (0.00s async, 0.02s sync)
2 tests, 0 failures


Seems to be working for me:

  test "greets the world in spanish" do
    :debugger.start()
    :int.ni(DebugerTest)
    :int.break(DebugerTest, 3)
    assert DebugerTest.greet("Hola") == :fail_greet
  end

and then:

iex -S mix test --trace

waits for the debugger to finish.

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