[release-branch.go1.9] internal/poll: add tests for Windows file and serial ports

[release-branch.go1.9] internal/poll: add tests for Windows file and serial ports

I also wanted to test net sockets, but I do not know how to
access their file handles. So I did not implement socket tests.

Updates #21172

Change-Id: I5062c0e65a817571d755397d60762c175f9791ce
Reviewed-on: https://go-review.googlesource.com/53530
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-on: https://go-review.googlesource.com/71131
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>