cmd/dist: allow builders to control granularity of test/ directory sharding

Even with 10 shards on builders, it still takes about ~2.5 minutes per
shard (and getting slower all the time as the test/ directory grows).
I'm currently experimenting with massively sharding out testing on
Cloud Run (each dist test & normal TestFoo func all running in
parallel), and in such a setup, 2.5 minutes is an eternity. I'd like
to increase that dist test's sharding from 10 to more like 1,000.

Updates golang/go#31834

Change-Id: I8b02989727793b5b5b2013d67e1eb01ef4786e28
Reviewed-on: https://go-review.googlesource.com/c/go/+/175297


Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>