Blender Git Loki

Tests: Add render tests for Cycles

The idea is to use the set of really small images from the lib folder
and run Cycles render on them comparing render output to reference
images in the tests repository.

For sure same thing could become more generic for BI or Freestyle
render engines.

This is not totally easy to use at this moment, basically because of
a bit of voodoo mango jambo black magic happening around how we are
dealing with addons for ctests. Main issue is that blender loads all
the scripts from the source directory, meaning it can't pick cycles
up.

Once this is figured out in a clean fashion the branch will be merged
to master. For until then all the tweaks are gonna to be done here.

For now it's possible to set BLENDER_USER_SCRIPTS to the directory
with Cycles addon so it is picked up nicely.

Reviewers: campbellbarton

Differential Revision: https://developer.blender.org/D1011