windows: fix test causing checkptr failure
windows: fix test causing checkptr failure

TestNtCreateFileAndNtSetInformationFile is failing checkptr. Fix by
using replacing [:] with [:n:n].

Fixes golang/go#49217

Change-Id: I08e7ed41b33452c096cf40cdbf9d73989b769bdb
Reviewed-on: https://go-review.googlesource.com/c/sys/+/359496
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Alex Brainman <54ba58c8441b5eff861de6b4e5ee46baf086b142@gmail.com>
Trust: Alex Brainman <54ba58c8441b5eff861de6b4e5ee46baf086b142@gmail.com>
Trust: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>

golang/sys,golang/sys,golang/sys