cbuildbot: Teach the HWTestStage to launch suites using subsystems

Teach the HWTestStage to launch suites based on the subsystems that the
relevant changes to the build test on.

BUG=chromium:515688
TEST=Pass unittest

Change-Id: If9102991a2ab5ab9f8c79ca7c8b9d587ed6c7b08
Reviewed-on: https://chromium-review.googlesource.com/283567
Trybot-Ready: Shuqian Zhao <shuqianz@chromium.org>
Tested-by: Shuqian Zhao <shuqianz@chromium.org>
Reviewed-by: Shuqian Zhao <shuqianz@chromium.org>
Commit-Queue: Shuqian Zhao <shuqianz@chromium.org>
3 files changed