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 <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

Author: 0intro

src/flag/flag_test.go

@@ -13,6 +13,7 @@ import (
 	"io/ioutil"
 	"os"
 	"os/exec"
+	"runtime"
 	"sort"
 	"strconv"
 	"strings"
@@ -599,6 +600,10 @@ func TestExitCode(t *testing.T) {
 		)
 		cmd.Run()
 		got := cmd.ProcessState.ExitCode()
+		// ExitCode is either 0 or 1 on Plan 9.
+		if runtime.GOOS == "plan9" && test.expectExit != 0 {
+			test.expectExit = 1
+		}
 		if got != test.expectExit {
 			t.Errorf("unexpected exit code for test case %+v \n: got %d, expect %d",
 				test, got, test.expectExit)