Summary:
A new namespace KWin::Test is added which provides a few helper
functions. It makes it easy to setup a KWayland client connection with
the base set to be able to create a Surface and flags to create
additional interfaces. This replaces the KWayland connection dance in
init() methods. For cleanup() there is also a dedicated helper function.

In addition there are helper functions to:
* render a surface
* create a surface
* create a shell surface
* flush the wayland client connection
* access to the created interfaces - for compatibility with existing code

The idea is to extend this Test library also for other common use cases
like creating an X11 connection and X11 windows, etc.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D2053