Tests: implement system test framework

Tests: implement system test framework

As discussed at length, this is the implementation of the new system
tests framework for shadow. This is a proof of concept that contains the
key elements to be able to run basic user (i.e. useradd, usermod) and
group (i.e. usermod) tests. If you like the framework the rest of the
functionality will be added in the future.

Some useful facts:
* It is implemented in python
* It is based on pytest and pytest-mh
* It works on all the distributions that are part of our CI
* It can be run in the cloud (VM or container) as well as on-premises
* After the execution of each test the environment is cleaned up
* Logs and other artifacts for failed tests are collected
* It has a rich API that can be extended and extended to cover new
  functionalities

Closes: https://github.com/shadow-maint/shadow/issues/835
Signed-off-by: Iker Pedrosa <ipedrosa@redhat.com>