os: skip TestPipeThreads on GOOS=darwin

Updates #32326.
Updates #33953.

Change-Id: I97a1cbe682becfe9592e19294d4d94f5e5b16c21
Reviewed-on: https://go-review.googlesource.com/c/go/+/192342
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/os_test.go

@@ -2251,6 +2251,8 @@ func TestPipeThreads(t *testing.T) {
 		t.Skip("skipping on Plan 9; does not support runtime poller")
 	case "js":
 		t.Skip("skipping on js; no support for os.Pipe")
+	case "darwin":
+		t.Skip("skipping on Darwin; issue 33953")
 	}
 
 	threads := 100