Currently all tests in a PRESUBMIT.py file are run in parallel, but not
all tests across PRESUBMIT.py files.
This introduces a flag that will allow presubmit to run all tests across
PRESUBMIT files in parallel.
Bug: 819774
Change-Id: Idd3046cb3c85e9c28932a9789ba7b207a01d9f99
Reviewed-on: https://chromium-review.googlesource.com/994241
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>