add some tests for corrupt configs
Summary:
This allows us to make sure that when a config file is borked, we're not
suddenly creating invalid configs.
Test Plan: tests pass
Reviewers: Plasma, graesslin
Reviewed By: Plasma, graesslin
Subscribers: broulik, plasma-devel
Projects: Plasma
Differential Revision: https://phabricator.kde.org/D1535