This CL implements two features: first off we can now split the tests
into several shards which we can run independently on different
machines.

Second it implements a simple retry mechanism that will retry a failed
test a number of times and count any first passing test as a pass.

Bug: angleproject:5875
Bug: angleproject:5966
Change-Id: Ieef4ea0e4eebdb286a32de3b1e3fef4f2eda0600
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2897546Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>