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/22357Reviewed-by: Robert Griesemer <gri@golang.org>

src/time/time.go

@@ -476,7 +476,7 @@ func (d Duration) String() string {
 		w--
 		switch {
 		case u == 0:
-			return "0"
+			return "0s"
 		case u < uint64(Microsecond):
 			// print nanoseconds
 			prec = 0

src/time/time_test.go

@@ -533,7 +533,7 @@ var durationTests = []struct {
 	str string
 	d   Duration
 }{
-	{"0", 0},
+	{"0s", 0},
 	{"1ns", 1 * Nanosecond},
 	{"1.1µs", 1100 * Nanosecond},
 	{"2.2ms", 2200 * Microsecond},