testing: with -benchtime=1x, run the benchmark loop exactly once

Like with -benchtime=1ns, if we find that the "discovery" round (run1)
has already crossed the -benchtime threshold, we skip running more
iterations.

Fixes #32051

Change-Id: I76aaef2ba521ba8ad6bbde2b14977e191aada5e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/331089


Trust: Caleb Spare <cespare@gmail.com>
Run-TryBot: Caleb Spare <cespare@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>