Help track down errors and other unwanted output in ExUnit tests when `mix test --trace` isn't sufficient

test_reporter.ex

defmodule TestReporter do
  # Why: This is useful for tracking down intermittent errors and output in tests.
  # What: Reports the test that ran in the test suite, in the order that they ran, and includes each test's ordinal number
  # in the order of the test's finish time.
  # 
  # Usage:
  # 1. Save this file to `tests/support` or anywhere it'll get picked up by the Elixir compiler.
  # 2. In test_helper.ex, add this GenServer as an ExUnit formater, like this:
  #    `ExUnit.start(exclude: [:external], formatters: [ExUnit.CLIFormatter, TestReporter])`
  # 3. Run the test suite. At the end of the run, it will list all the tests that ran, in the order that they ran.

  use GenServer

  def start_link(init_args) do
    # you may want to register your server with `name: __MODULE__`
    # as a third argument to `start_link`
    GenServer.start_link(__MODULE__, [init_args])
  end

  def init(_config) do
    {:ok, []}
  end

  def handle_cast({:test_finished, test}, tests) do
    {:noreply, [test | tests]}
  end

  def handle_cast({:suite_finished, _time}, tests) do
    for {test, index} <- Enum.with_index(tests) do
      IO.puts("#{index + 1}: #{test.module}: #{test.name}")
    end

    {:noreply, tests}
  end

  def handle_cast({_other, _data}, state) do
    {:noreply, state}
  end
end