os: force consistent mtime before running fstest on directory on Windows

FindFileNext sometimes returns a different mtime than looking at the
file directly, because the MFT on NTFS is written to lazily. In order to
keep these in sync, we use GetFileInformationByHandle to get the actual
mtime, and then write it back to the file explicitly.

Fixes #42637.

Change-Id: I774016d3ac55d0dc9b0f9c1b681516c33ba0d28a
Reviewed-on: https://go-review.googlesource.com/c/go/+/285720


Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>