Blender Git Loki

Cycles: add unit tests for supported constant folding rules.

Code coverage of different combinations of secondary conditions
is obviously not complete because there are so many of them, but
all main rules should be there.

The reason for CORRECT vs INVALID is that both words have the same
number of characters so calls line up, but look quite different.

Reviewers: #cycles, sergey

Reviewed By: #cycles, sergey

Subscribers: dingto, sergey, brecht

Differential Revision: https://developer.blender.org/D2130