Multiple tests are run in paralell to squeeze the most out of CPUs.
Tests are stored in a work queue. Whenever a CPU becomes available,
the next unrun test is grabbed and run on a spawn process on that CPU.

Each cpu gets their own environment and build folders and trace folders
so that tests don't overwrite each other.

Bug: angleproject:4817
Change-Id: Ifd35c9c75522e480b0257d090d5af70f2a3428ba
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2296040
Commit-Queue: Manh Nguyen <nguyenmh@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>