net: reenable a previously skipped test

Update #8859

Change-Id: I5b0005b308e83954a495f06d27b7d8d30e813820
Reviewed-on: https://go-review.googlesource.com/8193Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/tcp_test.go

@@ -494,13 +494,11 @@ func TestTCPConcurrentAccept(t *testing.T) {
 
 func TestTCPReadWriteAllocs(t *testing.T) {
 	switch runtime.GOOS {
-	case "nacl", "windows", "darwin", "dragonfly":
+	case "nacl", "windows":
 		// NaCl needs to allocate pseudo file descriptor
 		// stuff. See syscall/fd_nacl.go.
 		// Windows uses closures and channels for IO
 		// completion port-based netpoll. See fd_windows.go.
-		// Darwin is unreliable for unknown reasons (issue 8859).
-		// Dragonfly also unreliable (lumped into issue 8859).
 		t.Skipf("not supported on %s", runtime.GOOS)
 	}