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
http://codereview.appspot.com/6849097

glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,webfd/go-zh,mhennings/marcohennings-go,d0f/go-zh,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,d0f/go-zh,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,rdp/rogerpack2005-golang,mhennings/marcohennings-go,bryanxu/go-zh,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,webfd/go-zh,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,scirelli/scirelli-go,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,scirelli/scirelli-go,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,webfd/go-zh,bryanxu/go-zh,webfd/go-zh,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,webfd/go-zh,rdp/rogerpack2005-golang,scirelli/scirelli-go,sanjosh/sanjos100-tipc,scirelli/scirelli-go,scirelli/scirelli-go,bryanxu/go-zh,mhennings/marcohennings-go,d0f/go-zh,scirelli/scirelli-go,webfd/go-zh