cmd/bent: shuffle runs; add flag for grouping by benchmark

also rename never-used "-S" flag "-sandbox"

Change-Id: Ie0ee9b242b179ebb3dd76ce3cf2143ea8f215d53
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/565116
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>