Summary:
The maximise test is moved and a simple test is added for smart
placement and placeCorner.
The class tries to make a framework to make it faster to add future
xdg_toplevel placement tests without having to copy too much
boilerplate.
Test Plan: Passes
Reviewers: #kwin, zzag
Reviewed By: #kwin, zzag
Subscribers: zzag, kwin
Tags: #kwin
Differential Revision: https://phabricator.kde.org/D21996
