time: during short test, do not bother tickers take longer than expected

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5752058

src/pkg/time/tick_test.go

@@ -22,7 +22,7 @@ func TestTicker(t *testing.T) {
 	dt := t1.Sub(t0)
 	target := Delta * Count
 	slop := target * 2 / 10
-	if dt < target-slop || dt > target+slop {
+	if dt < target-slop || (!testing.Short() && dt > target+slop) {
 		t.Fatalf("%d %s ticks took %s, expected [%s,%s]", Count, Delta, dt, target-slop, target+slop)
 	}
 	// Now test that the ticker stopped