crypto/rand: use io.ReadFull in test
crypto/rand: use io.ReadFull in test

On Solaris versions before Solaris 11, the kernel will not
return more than 1040 on a single read from /dev/urandom.

R=golang-dev, agl, bradfitz, rsc, iant, dchest
CC=golang-dev
http://codereview.appspot.com/6113046

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