This CL sorts tests into subprocesses organized by a single config.
It will allow for the test runner to use a single EGL display per
process in most cases. It doesn't implement the display sharing right
now, that'll be in a forthcoming CL. It assumes test configs are split
between / and (optional ending) __ characters.

Bug: angleproject:3162
Change-Id: I28e8b4687640da6e501f08b8b9347185c99ce30f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2280403Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>