[autotest/integration] Add helper for waiting for a shown ShellClient
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