os/exec: pass ExitCode tests on Plan 9
os/exec: pass ExitCode tests on Plan 9

Fixes #27294

Change-Id: I8db5ca0f0c690bf532d3d33b8ed7d2633ad1702b
Reviewed-on: https://go-review.googlesource.com/131855
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/go,akutz/go,nawawi/go,akutz/go,akutz/go,akutz/go,golang/go,CAFxX/go,golang/go,josharian/go,cloudflare/go,cloudflare/go,nawawi/go,josharian/go,CAFxX/go,nawawi/go,akutz/go,nawawi/go,CAFxX/go,josharian/go,cloudflare/go,akutz/go,cloudflare/go,josharian/go,cloudflare/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,josharian/go,golang/go,CAFxX/go,golang/go,nawawi/go,akutz/go,akutz/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go,josharian/go,josharian/go,golang/go,josharian/go,CAFxX/go