testing: fix example test fd leak
testing: fix example test fd leak

Close the read side of the pipe.
Fixes issue 4551.

R=rsc
CC=golang-dev
https://codereview.appspot.com/6962049

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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