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://golang.org/cl/7363056