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