When grouping tests to test batches, instead of grouping neighboring
tests together, use striping so that slow tests are fanned out
into multiple batches. This way, no test batch is too slow.

Bug: angleproject:4817
Change-Id: I7825539e401ea53b5a31167f206d1f5bfb07bdbf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2339540Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Manh Nguyen <nguyenmh@google.com>