test/mapnan: use time.Now instead of syscall.Getrusage
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://codereview.appspot.com/15570046

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang