When you mix test your umbrella, you end up running the entire test suite. Even though
mix test apps/child_app/test/child_app_test.exs # don't 😢
technically works, it is still running from inside the umbrella app so this command runs for every app inside the umbrella.
$ mix cmd --app child_app mix test test/child_app/child_app_test.exs # do 😀