tst_QWidget/XCB: Use bottom/right corner of screen as safe cursor position.

Pushing the mouse rapidly to the top left corner causes KDE / KWin to
switch to "Present all Windows" (Ctrl+F9) mode, showing all windows.
This has been observed to be triggered programmatically by
QCursor::setPos(0, 0) if there is no other window beneath and apparently
depending on the perceived mouse "speed".
Suppress this by using the bottom right corner for XCB.

Change-Id: Id18d2f45a095ed4d4f365f010cf45a20b0d9435e
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@qt.io>