The constant folding test classes are moved into a separate file in
test_utils. This will enable adding multiple test files that use
constant folding test classes, so that constant folding tests can be
organized better.

TEST=angle_unittests
BUG=chromium:661857

Change-Id: I00bf25a4b941bdc1364ff5aa9bee2d571e4b0ea0
Reviewed-on: https://chromium-review.googlesource.com/414910Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>