Fix qCompare between strings.

Summary:
Hotfix for a potential incompatibility with old Qt?
The test code was qCompare'ing a QString with a const char pointer, and depending on the platform, was successfully casting between those, or not.

Test Plan: None, this fixes a compile issue.

Reviewers: mutlaqja

Reviewed By: mutlaqja

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D27524