[autotests] Fix race condition in ShellClient::testUnresponsiveWindow

Summary:
The test executable "kill" freezes itself after 1ms, supposedly after
showing a window.

However showing a window is not syncronous on wayland, it's illegal to
map a buffer before getting a configure event from the server.

This patch removes any potential for a race by having the server tell
our test executable when to freeze.

Test Plan: Test still passed

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D19406