Convert TestPythonBase to a true base class that only provides code
implementing various tests without defining tests themselves in a
"discoverable" form.

To retain correct testing functionality, add TestPython2 derived class
that uses code from TestPythonBase to define actual runnable test.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Reviewed-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Tested-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
[Thomas: fix typo in commit log, update .gitlab-ci.yml, both pointed
by Ricardo.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>