sweet: add a short mode for testing

Right now the full benchmark suite takes quite a while to run once:
up to 20 minutes. This change introduces a short mode that reduces that
down to about 4 minutes and 30 seconds. With some judicious sharding,
that can likely be brought down further.

Change-Id: Idf2e6724347bc86450bff904bde7161312bd0cc3
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/378275
Reviewed-by: Michael Pratt <mpratt@google.com>
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
13 files changed