os: skip Root.Chtimes atime check on netbsd, truncate a/mtime on plan9
The NetBSD builder has noatime set on its filesystem.
Skip testing the atime on this builder.
Plan9 has second precision on its atime and mtimes.
Truncate the values passed to Chtimes.
For #72957
Change-Id: I963e2dd34075a9ba025e80641f0b675d5d912188
Reviewed-on: https://go-review.googlesource.com/c/go/+/659356
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Ian Lance Taylor <iant@google.com>