os/exec: make sure file is not closed early in leaked fd test
os/exec: make sure file is not closed early in leaked fd test

Without this change, fd3 can be collected by the garbage
collector and finalized, which causes the file descriptor to
be closed, which causes the call to os.Open to return 3 rather
than the expected descriptor number.

R=golang-dev, gri, bradfitz, bradfitz, iant
CC=golang-dev
http://codereview.appspot.com/5607056

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