Revert "presubmit support: Run all tests in parallel."

This reverts commit c7d0b340.

Reason for revert:

See https://logs.chromium.org/v/?s=infra%2Fbuildbucket%2Fcr-buildbucket.appspot.com%2F8950238735438551408%2F%2B%2Fsteps%2Fpresubmit%2F0%2Fstdout
Two go bootstraps executing concurrently and breaking each other. govet and golint presubmit checks both try to bootstrap go at the same time. 


Original change's description:
> presubmit support: Run all tests in parallel.
> 
> 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>

TBR=agable@chromium.org,ehmaldonado@chromium.org

Change-Id: Ia5b5ae5af8d6cf9bd72388f58ff0f032a4367e10
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 819774
Reviewed-on: https://chromium-review.googlesource.com/994032
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>