net: disable TestTCPReadWriteAllocs on dragonfly

Update #8859
Update #10042

Change-Id: Idc7eadb447b73563ce9085e50c2042652442c2d9
Reviewed-on: https://go-review.googlesource.com/6412Reviewed-by: Minux Ma <minux@golang.org>

src/net/tcp_test.go

@@ -494,12 +494,13 @@ func TestTCPConcurrentAccept(t *testing.T) {
 
 func TestTCPReadWriteAllocs(t *testing.T) {
 	switch runtime.GOOS {
-	case "nacl", "windows", "darwin":
+	case "nacl", "windows", "darwin", "dragonfly":
 		// 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)
 	}