wtest.lua adapted for use with issue #372

_wtest.lua adapted for use with [issue 372](https://github.com/koreader/koreader/issues/372#issuecomment-39000725)._

1. wtest.lua crashed. So I cut&paste some lines from reader.lua to fix i...t.

2. I added the function TestVisible to assess the extreme coordinates still visible on the Kobo Aura.