os: do not close syscall.Stdin in TestReadStdin

By calling NewConsoleFile on syscall.Stdin, we wind up closing it when
the function returns, which causes errors when all the tests are run in
a loop. To fix this, we instead create a duplicate handle of stdin.

Fixes #43720.

Change-Id: Ie6426e6306c7e1e39601794f4ff48bbf2fe67502
Reviewed-on: https://go-review.googlesource.com/c/go/+/284140


Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>