time: print zero duration as 0s, not 0

time: print zero duration as 0s, not 0

There should be a unit, and s is the SI unit name, so use that.
The other obvious possibility is ns (nanosecond), but the fact
that durations are measured in nanoseconds is an internal detail.

Fixes #14058.

Change-Id: Id1f8f3c77088224d9f7cd643778713d5cc3be5d9
Reviewed-on: https://go-review.googlesource.com/22357
Reviewed-by: Robert Griesemer <gri@golang.org>