Use environment variable to know where the config file is

This branch adds an interesting option which will allow us, as developers
have our own configuration file in a different directory than the project.

We really need this, particularly because the changes we make on this file
always get in the middle of our work and commits, and we end up running
`git stash` and `git stash pop` between checkouts and pulls.

Not only this branch is useful because of that, but also it has been a good
chance to introduce an important new bit of the test infrastructure.