[GH #400] Flatten structure of test/extension/io/output/ folder

From: Mateusz Loskot (mateusz_at_[hidden])
Date: 2019-10-29 08:02:00

Hi,

FYI, I also applied this last minute tweak to test images structure,
it has been merged to develop and ready for release with 1.72

https://github.com/boostorg/gil/pull/400

Best regards,
-- 
Mateusz Loskot, http://mateusz.loskot.net