How to run unit tests for the whole project in python?

According to this article http://ift.tt/1pzoXPJ

the tests for as package should be in a directory inside the package.

For example if the package is named twisted we would have a another directory twisted/test in which we would have our tests.

I am not really sure how I should write my tests for every source code script and how to run all my tests at once ( also for more than one packages).

A link to a simple example project in GitHub would be most helpful :)