os/exec: TestExitCode failing on Plan 9

[0intro]

CL 125443 added TestExitCode. However, this test is failing on Plan 9, because Plan 9 programs return a string and not an exit code.

--- FAIL: TestExitCode (0.05s)
    exec_test.go:178: ExitCode got 1, want 42
    exec_test.go:186: ExitCode got 1, want 2
    exec_test.go:194: ExitCode got 1, want 255
FAIL
FAIL	os/exec	1.426s

See https://build.golang.org/log/8411cb50158709f843f78d9c6634bf005b6dc1fb

[fhs]

It should just accept those results on Plan 9. It doesn't seem worth it to parse the int out. Even then, exit "0" would be a special case.

[gopherbot]

Change https://golang.org/cl/131855 mentions this issue: os/exec: pass ExitCode tests on Plan 9