Update wtest.lua

The Kobo Aura screen's outer sides are partially obscured by the bezel. With wtest the minimum and maximum x and y values are determined which are still visible. The visible screen is x=3..754 and y=3..1009.

__commit 1__ Added needed requires and path from `reader.lua` to `wtest.lua` to prevent crashing of `wtest.lua`.
__commit 2__ Offsets of 3 for `widget.x` and `widget.y` are introduced. 
__commit 3__ The `Screen.GetSize`, `Screen.GetWidth` and `Screen.GetHeight` functions are hardcoded at 751x1006.
__commit 4__ Introduces rotation dependency for the new functions.
__commit 5__ Finetuning of the screen dimensions to 757x1012

This solves #372 and only affects Kobo Aura's