test/mapnan: use time.Now instead of syscall.Getrusage

Avoids a dependency on a somewhat nonstandard part of package syscall.

R=golang-dev, dave, r
CC=golang-dev
https://golang.org/cl/15570046

test/mapnan.go

@@ -13,17 +13,13 @@ import (
 	"fmt"
 	"math"
 	"time"
-	"syscall"
 )
 
 func main() {
 
 	// Test that NaNs in maps don't go quadratic.
 	t := func(n int) time.Duration {
-		var u0 syscall.Rusage
-		if err := syscall.Getrusage(0,  &u0); err != nil {
-			panic(err)
-		}
+		t1 := time.Now()
 		m := map[float64]int{}
 		nan := math.NaN()
 		for i := 0; i < n; i++ {
@@ -32,11 +28,7 @@ func main() {
 		if len(m) != n {
 			panic("wrong size map after nan insertion")
 		}
-		var u1 syscall.Rusage
-		if err := syscall.Getrusage(0,  &u1); err != nil {
-			panic(err)
-		}
-		return time.Duration(u1.Utime.Nano() - u0.Utime.Nano())
+		return time.Since(t1)
 	}
 
 	// Depending on the machine and OS, this test might be too fast