os: enable TestGetppid on Plan 9

Fixes #8206.

Change-Id: Iec1026ecc586495f5c9562cc84b3240c71d53da5
Reviewed-on: https://go-review.googlesource.com/22164
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Author: 0intro

src/os/os_test.go

@@ -1705,11 +1705,6 @@ func TestKillStartProcess(t *testing.T) {
 }
 
 func TestGetppid(t *testing.T) {
-	if runtime.GOOS == "plan9" {
-		// TODO: golang.org/issue/8206
-		t.Skipf("skipping test on plan9; see issue 8206")
-	}
-
 	testenv.MustHaveExec(t)
 
 	if Getenv("GO_WANT_HELPER_PROCESS") == "1" {