[release-branch.go1.13] 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>
(cherry picked from commit bac5b3f0fe7838ecf6e206fa8d2123c7771eb976)
Reviewed-on: https://go-review.googlesource.com/c/go/+/192757
Run-TryBot: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/src/os/os_test.go b/src/os/os_test.go
index c101281..6c88d7e 100644
--- a/src/os/os_test.go
+++ b/src/os/os_test.go
@@ -2251,6 +2251,8 @@
 		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