testing: reference benchmark format spec

This updates the testing package documentation to reference the
specification for the benchmark format, and points users to our
standard tools for working with benchmark data. (It's somewhat
remarkable how widely used benchstat appears to be given that we don't
mention it anywhere!)

Change-Id: Idbb4416d2fde9606ea7e6c15595f3b9e6a38f3b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/357589


Trust: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>