runtime: fix flakiness on futexsleep test
runtime: fix flakiness on futexsleep test

Fixes issue 7496.

LGTM=jsing
R=golang-codereviews, jsing
CC=golang-codereviews
https://codereview.appspot.com/72840043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang