os/exec: Pass tests on Plan 9
os/exec: Pass tests on Plan 9

Adjust the exit status string for Plan 9.

Upon allocating >100 file descriptors, Plan 9
raises a warning. Moreover, the Go runtime for
32-bit version of Plan 9 keeps /dev/bintime
open for its implementation of runtime.nanotime().
This change accounts for these things in
TestExtraFiles.

R=rsc, rminnich, ality, bradfitz
CC=golang-dev
https://codereview.appspot.com/7363056

Committer: Anthony Martin <35eacf30aabdab61efbd37765ec632fc68b70c11@pbrane.org>

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