Cycles ctests: avoid having runtime check script

Use command line argument and output parse to do checks which
used to be done with the separate script.

Add BLENDER_VERBOSE env var check to cycles test

This means you can see Blender's output if you want.
also minor style edits

Cycles ctests: Use hardcoded locations to idiff on osx/win

The idea is to have this binary in our libs folder, so it's easy
to run tests on those platforms without installing anything extra.

Actual binaries are to be uploaded still tho.

Cycles ctests: Automatically find idiff application

Cycles ctests: Add runtime check for Cycles

The idea is to check scene's render engine and if it's not Cycles
report about this and stop running tests (because it's not likely
Cycles will be loaded correctly).

This way we'll fail Cycles tests real quick without install target
and give quite nice error message about that.

Cycles ctests: Run blender without system script override

The idea is to force using Cycles tests only after running install target.
More i'm thinking about ways to not require this more it's clear there is
no clear way to support this. It is possible to hack something around and
make tests to be running smoothly, but it's never gonna to give all the
features (like GPU which required source file on linux and compiled cubin
on windows, OSL which requires compiled script) without any manual tweaks
anyway.

So let's be damn stupid and not try to be smart and just assume install
target was run which makes Cycles tests just happy without any extra
setup. Would be nice to detect and report missing target install so we
don't spend time on rendering Cycles scenes with Blender Internal.

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