TL;DR

You need inotify-tools installed before going further.

sudo apt-get install inotify-tools

You can then execute the following command to automatically reload your tests when updating a file inside web/ lib/ or test/.

inotifywait -m -r -e modify,attrib,close_write,move,create,delete \

  test \

  lib \

| MIX_ENV=test mix test --trace --stale --listen-on-stdin

Happy Coding!