os/exec: deflake a test on Linux
os/exec: deflake a test on Linux

Work around buggy(?) Linux /proc filesystem.

Fixes Issue 7808

LGTM=iant
R=golang-codereviews, iant
CC=adg, golang-codereviews
https://codereview.appspot.com/90400044

wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old