os: re-enable TestPipeThreads on darwin

CL 197938 actually fixes those regression on Darwin as syscalls
are no longer labeled as always blocking and consume a thread.

Fixes #33953
Fixes #32326

Change-Id: I82c98516c23cd36f762bc5433d7b71ea8939a0ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/199477
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

src/os/os_test.go

@@ -2254,8 +2254,6 @@ 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