[autotests] Use Xwayland class in WaylandTestApplication

Summary:
By using the new Xwayland class we can reduce code duplication.

An abstract parent class is introduced, that allows interfacing
with the Xwayland class from the test binaries.

Test Plan: Autotests still pass.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D15022