os: skip TestExecutableDeleted on plan9

This test deletes the running executable file, which is not safe
on Plan 9. The test was working by accident prior to commit 02ab8d1a,
which must have changed the page reference ordering just enough to
cause a new demand page-in after deletion of the file.

Fixes #45941

Change-Id: Ic13d8032c21cee603e3afc3c5c4da6093fc37bf4
Reviewed-on: https://go-review.googlesource.com/c/go/+/316829


Reviewed-by: David du Colombier <0intro@gmail.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: David du Colombier <0intro@gmail.com>
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>