Revert "os: enable TestGetppid on Plan 9"

This reverts commit a677724e.

Change-Id: I6a54ac26a6deca5b2a39ec9f899469a88b543d3d
Reviewed-on: https://go-review.googlesource.com/22980Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/os_test.go

@@ -1705,6 +1705,11 @@ 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" {