flag: fix TestExitCode on Plan 9

CL 221427 added TestExitCode. This test is failing
on Plan 9 because ExitCode is always equal to 1
on error since Plan 9 use error strings.

This change fixes TestExitCode by checking that
ExitCode is equal to 1 on error instead of the
specific value.

Fixes #38237.

Change-Id: Ie269722e731e275e5bfc51644c1fa6be76525f1f
Reviewed-on: https://go-review.googlesource.com/c/go/+/227158


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