TestDup used a file descriptor without ensuring it was free,
leading to rare crashes in the runtime netpoller when the victim fd
was the polling descriptor.

Updates golang/go#29423

Change-Id: Idc8b6b47f7e966e045f57f2028e7b6b79e0fb3f3
Reviewed-on: https://go-review.googlesource.com/c/163638Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>