Works similar to regular Cycles tests, just does OpenGL render to
get output image.
Seems to work fine with the only funny effect: Blender window will
pop up for each of the tests. This is current limitation of our
OpenGL context. Might be changed in the future.
Made them closer to how GTest shows the output, so reading test logs
is easier now (at least feels more uniform).
Additionally now we know how much time tests are taking so can tweak
samples/resolution to reduce render time of slow tests.
It is now also possible to enable colored messages using magic
CYCLESTEST_COLOR environment variable. This makes it even easier to
visually grep failed/passed tests using `ctest -R cycles -V`.
This reverts commit d390e24c49.
Forcing tests to success is really bad idea. It'll only lead to cases when
you see PASSED and will think everything is OK.
Long story short: never force tests to pass!
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.
Thanks Campbell for review and code tweaks!
