A mix dependency is included and compiled in its :prod environment, by default. If you're not using umbrellas and have sibling dependent modules that should share testing support resources, use the :env option:

defp deps do
  [
    { :my_module, path: "../my_module", env: Mix.env() }
  ]
end

If my_modue compiles support test modules, they will be available to the current application during testing.