Customize the test command

[4dsr]

Hi,
The support for testing feature seems to be a great thing. Unfortuantely i can not use it in my project because it is umbrella project running in the containers. Thus my mix test command looks like this: docker exec -it -e MIX_ENV=test $container mix cmd --app $app mix test
There is a need to customize test command for particular projects based on different factors.
Giving the ability for the user to transform test command to desired format could address these needs. That could be a simple function kept in the project root dir in a hidden file for example.