Fix scroll direction as UP was going down (and DOWN up).

When FxRobot has been refactored, Up and Down scroll have been inverted.

Old code : 
     
    /**
     * Scrolls the mouse-wheel a given number of notches in a direction.
     *
     * @param amount    the number of notches to scroll
     * @param direction
     * @return
    */
    public GuiTest scroll(int amount, VerticalDirection direction) {
        for (int x = 0; x < Math.abs(amount); x++) {
           screenRobot.scrollMouse(directionToInteger(direction));
        }
        return this;
     }
    
    private int directionToInteger(VerticalDirection direction) {
        if (direction == VerticalDirection.UP)
            return -1;
         return 1;
     }

Actual code is : 

    @Override
    public void scroll(int amount) {
        if (amount >= 0) {
            scrollUp(amount);
        }
        else {
            scrollDown(Math.abs(amount));
        }
    }
    [...]
    @Override
    public void scrollUp(int positiveAmount) {
        for (int scrollTick = 0; scrollTick < positiveAmount; scrollTick++) {
            screenRobot.scrollMouse(1);
        }
    }

    @Override
    public void scrollDown(int positiveAmount) {
        for (int scrollTick = 0; scrollTick < positiveAmount; scrollTick++) {
            screenRobot.scrollMouse(-1);
        }
    }


Regression test added (I hope the Duke with a guitar image do not have a copyright).