Test that the user in python and not root

[hillairet]

We have seen a couple of times the situation in the images built with this base where the user is switched to root using USER root to install packages then not switched back to python.
Our test suite can enforce better practices and test that the user is not root.

The command whoami returns the username.
Add a check to enforce the user of the docker image to be python in the test_suite.sh.

[hillairet]

Done in #304