time: add Now()/UnixNano() malloc tests

time: add Now()/UnixNano() malloc tests

The fix for issue 4403 may include more calls to time.Now().UnixNano(). I was concerned that if this function allocated it would cause additional garbage on the heap. It turns out that it doesn't, which is a nice surprise.

Also add benchmark for Now().UnixNano()

R=bradfitz, minux.ma
CC=golang-dev
https://golang.org/cl/6849097