Testrunner runs variants of a test sequentially without taking sharding into
account. A slow test with slow variants slows down the whole test run no matter
the sharding configuration.

This CL implements a test hashing algorithm and variant sharding for test
variants.

R=machenbach@chromium.org
CC=yangguo@chromium.org,sergiyb@chromium.org

Bug: v8:8174
Change-Id: I15f8c547fa2f361fb6c53bf8d5df055d3df38d3e
Reviewed-on: https://chromium-review.googlesource.com/c/1458016
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59530}