os/exec: fix number of expected file descriptors on Plan 9
os/exec: fix number of expected file descriptors on Plan 9

Since CL 104570043 and 112720043, we are using the
nsec system call instead of /dev/bintime on Plan 9.

LGTM=rsc
R=rsc
CC=aram, golang-codereviews
https://codereview.appspot.com/155590043

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go