fix(autotests): fix simpleWrite test in kwaylandbackendtest

Summary:
The test still fails on removeOutput test, but that's fixed by
D27536.

Test Plan: simpleWrite test passes

Reviewers: Plasma, romangg

Reviewed By: Plasma, romangg

Subscribers: romangg, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27537