os: skip TestPipeThreads on Plan 9

It can't work since Plan 9 does not support the runtime poller.

Fixes build.

Change-Id: I9ec33eb66019d9364c6ff6519b61b32e59498559
Reviewed-on: https://go-review.googlesource.com/37091
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/os_test.go

@@ -1949,6 +1949,8 @@ func TestPipeThreads(t *testing.T) {
 		t.Skip("skipping on FreeBSD; issue 19093")
 	case "windows":
 		t.Skip("skipping on Windows; issue 19098")
+	case "plan9":
+		t.Skip("skipping on Plan 9; does not support runtime poller")
 	}
 
 	threads := 100