flag: TestExitCode failing on Plan 9

[0intro]

CL 221427 added TestExitCode. This test is failing on Plan 9.

flag provided but not defined: -x
flag needs an argument: -b
Usage of test:
  -b	usage
--- FAIL: TestExitCode (0.06s)
    flag_test.go:603: unexpected exit code for test case {flag:-undefined flagHandle: expectExit:2} 
        : got 1, expect 2
    flag_test.go:603: unexpected exit code for test case {flag:-h flagHandle:h expectExit:123} 
        : got 1, expect 123
    flag_test.go:603: unexpected exit code for test case {flag:-help flagHandle:help expectExit:123} 
        : got 1, expect 123
FAIL
FAIL	flag	0.094s

See https://build.golang.org/log/a1b96ba1f2c506abe814a4eddbf6335372c32d4d

[gopherbot]

Change https://golang.org/cl/227158 mentions this issue: flag: fix TestExitCode on Plan 9