time: skip a often-flaky test in short mode

In -test.short mode, skip measuring the upper bound of time
sleeps. The API only guarantees minimum bounds on sleeps,
anyway, so this isn't a bug we're ignoring as much as it is
simply observing bad builder virtualization and/or loaded
machines.

We keep the test in full mode where developers will
presumably be running on a lightly-loaded, native, fast
machine.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5713044

src/pkg/time/sleep_test.go

@@ -120,8 +120,11 @@ func TestAfterTick(t *testing.T) {
 	t1 := Now()
 	d := t1.Sub(t0)
 	target := Delta * Count
-	if d < target*9/10 || d > target*30/10 {
-		t.Fatalf("%d ticks of %s took %s, expected %s", Count, Delta, d, target)
+	if d < target*9/10 {
+		t.Fatalf("%d ticks of %s too fast: took %s, expected %s", Count, Delta, d, target)
+	}
+	if !testing.Short() && d > target*30/10 {
+		t.Fatalf("%d ticks of %s too slow: took %s, expected %s", Count, Delta, d, target)
 	}
 }