cartesian_config.py: add -r option for "repr mode"

This will allow us to generate output files in a format that can be parsed
using Python.

Signed-off-by: Eduardo Habkost <ehabkost@raisama.net>
cartesian_config: very simple unit test file

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
cartesian_config_unittest: Check using config file dump

Add the dump of a combination of very complex config files, and
check if the results match the config file dump exactly.

Signed-off-by: Eduardo Habkost <ehabkost@raisama.net>