Use QTEST_GUILESS_MAIN

The test still works fine with it, allowing it to run without a
display server.

Test Plan: Ran the test in an environment which doesn't allow X

Reviewers: #konsole, hindenburg

Subscribers: konsole-devel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D20067

(cherry picked from commit f1e7d896)