Are they containing scripts, or are you defining modules in them?

If they are defining proper modules, I tend to do it like this:

# mix.exs
defmodule Foo.MixProject do
  # ...

  def project do
    [
      # ...
      elixirc_path: compiler_paths(Mix.env())
    ]
  end

  def compiler_paths(:test), do: ["test/helpers"] ++ compiler_paths(:prod)
  def compiler_paths(_), do: ["lib"]

  # ...
end

Then I just create files with modules in them below test/helpers/, but as *.ex rather than *.exs. I haven’t had problems with this setup so far.

Also this picks up new files as you create them, ne need to remember to update them in a list. Ne need to remember correct path and filename, you can just use their modules as you would use a module from your application.