unittests: add unittests infrastructure

Borrowing `runtests.py` from avocado and creating a very simplified
(in a good way) set of utilities to run unittests that involve Django
code.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
unittests: add a first batch of unittests

This time dealing with the database models that power the application

Signed-off-by: Cleber Rosa <crosa@redhat.com>