When looking at the test results on build.kde.org we see this is the
test which fails most often and it's always at the same line of code.
So I just had a look with the thought: "what's special about this code,
why could it fail?"
Looking at it we start an external process which is supposed to connect
to KWin. We wait for the process to start and then wait for the shell
client added. This assumes that at the time we handle the wait for
started the connection of the window has not happened yet.
Thinking about it, it's possible that we get the wait for started after
we added the window. So this change tries to catch this special