testing: enable better loop time measurement for benchmarking.
With b.Loop() in place, the time measurement of loop scaling could be improved to be tighter. By identifying the first call to b.Loop(), we can avoid measuring the expensive ramp-up time by reset the timer tightly before the loop starts. The remaining loop scaling logic of b.N style loop is largely reused.
For #61515.
Change-Id: Ia7b8f0a8838f57c00ac6c5ef779d86f8d713c9b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/612835 Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>