Currently all tests in a PRESUBMIT.py file are run in parallel, but not
all tests across PRESUBMIT.py files.
This introduces a pool common to all files so we can run all of them.
Bug: 819774
Change-Id: Ic129af1bc9e6da568fa9fa71827193c6d8ab9af1
Reviewed-on: https://chromium-review.googlesource.com/973691
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>