[autotests] Test placement strategies

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