KCharSelect: add unittest. Hits the assert in QHeaderView.

Summary:
Just creating a KCharSelect instance was hitting that QHeaderView bug.
A unittest is a good way to make sure this doesn't happen again.

Test Plan: ctest

Reviewers: cfeck, aacid

Reviewed By: aacid

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D20515