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 f1e7d89667354626f6ef6b18f8b8734de130c8a0)