autotests: Make testX11Window quicker

Before

    Executed in   17.39 secs      fish           external
       usr time  422.91 millis    1.92 millis  420.99 millis
       sys time  586.41 millis    0.22 millis  586.19 millis

After

    Executed in    1.23 secs      fish           external
       usr time  395.63 millis    0.00 millis  395.63 millis
       sys time  228.55 millis    2.13 millis  226.42 millis