Makefile: Add gentestcases target

This calls gen_test_cases.sh in the proper environment.

Also, prevent gen_test_cases.sh from overriding test_file, to
provide stable signature (and avoid large git diff for no reason).

BRANCH=none
BUG=chromium:684354
TEST=make gentestcases -j8; git diff => no changes

Change-Id: I556285fd1a07a4d84f4ebd3fd7881ae06743716e
Reviewed-on: https://chromium-review.googlesource.com/439064
Commit-Ready: Nicolas Boichat <drinkcat@chromium.org>
Tested-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
2 files changed