QPixmap::grabWindow is a no-op with xcb qpa backend

Looking at qtbase f2b3dfd1f17952627fce9fde1264207958f87f43 , grabWindow seems to be a no-op in the xcb qpa backend.

QPixmap QXcbIntegration::grabWindow(WId window, int x, int y, int width, int height) const
{
    Q_UNUSED(window);
    Q_UNUSED(x);
    Q_UNUSED(y);
    Q_UNUSED(width);
    Q_UNUSED(height);
    return QPixmap();
}

I'm not sure how important this API is, but at least it causes some autotests to fail (e.g. tst_QPixmap::grabWindow).

It seems there is no API to query whether or not this feature is supported, or an API to query if xcb is the current qpa backend, so there's not really a correct way to skip the failing tests.