Summary:
Many tests create a Wayland window, render it and then wait till it's
created in KWin as a ShellClient. To reduce code duplication the test
helper provides helper methods to wait for the next ShellClient to be
shown and to directly render and wait for the window for that to be
shown.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D2057