[autotests] Try making KWinBindingsTest::testSwitchWindowScript more robust

Another random failure on build.kde.org (see T6546). This tries to use
QTRY_COMPARE on the SignalSpy's count instead of a wait in the hope that
this can handle the multi-threaded situation better. Though it might
also be possible that the other thread just didn't run in the 5 sec we
waited.