More flaky test fixes

default-size and restore-size has had some unintended failures it seems. I hope these two commits will make it better.

One is just a shot in the dark - the added "wait". I can't reproduce it myself. I suspect mutter will simply not see a window in time.

The other I have somewhat verified - I could reproduce it by running the same test a few hundred times in a row, but now instead I just seem to flood the X server as I get an general XIOError instead after running the same client test over and over.