Use QTEST_GUILESS_MAIN for testentry test

Summary:
The test doesn't need a Q{,Gui}Application and still works fine,
allowing it to run without a display server.

Test Plan: The test still passes fine with a running display server.

Reviewers: KDE Edu

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D23186