crypto/rand: add tests for Int, Prime
crypto/rand: add tests for Int, Prime

LGTM=rsc, dave
R=golang-codereviews, dave, josharian, rsc
CC=golang-codereviews
https://codereview.appspot.com/46490043

Committer: Dave Cheney <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@cheney.net>

rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,webfd/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,d0f/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,webfd/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,d0f/go-zh,d0f/go-zh,d0f/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,bryanxu/go-zh