Refactor constant folding tests 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/414910 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>