autotests: Make testPlasmaWindow quicker

Before

    Executed in   10.79 secs    fish           external
       usr time    4.85 secs  918.00 micros    4.85 secs
       sys time    0.62 secs  119.00 micros    0.62 secs

After

    Executed in    1.45 secs      fish           external
       usr time  941.63 millis  486.00 micros  941.14 millis
       sys time  578.99 millis   62.00 micros  578.93 millis