os: allow for variant plan9 error messages in TestOpenError
Different Plan 9 file servers may return different error strings
on an attempt to open a directory for writing: EISDIR, EACCES or
EPERM. TestOpenError allows for the first two, but it needs to
allow for EPERM as well.
Fixes #70440
Change-Id: I705cc086e21630ca254499ca922ede78c9901b11
Reviewed-on: https://go-review.googlesource.com/c/go/+/629635
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>