Autotest to simulate that no screens are available

Summary:
The test creates a Wayland display, locks the screen and destroys
the output. In this situation kscreenlocker_greet should not crash
or exit. The screen should stay locked.

Reviewers: Plasma, bshah

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2208