[wayland] Fix ShellClientTest

Summary:
When switching from maximised to full screen the geometry as kwin sees
it doesn't necessarily need to change, the test is wrong.

Also we can fix the XDGShell-deco tests.

wlshell-deco test still fails as before.

Test Plan: Tests pass \o/

Reviewers: KWin, graesslin

Reviewed By: KWin, graesslin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D16922