net/memnet: rename from net/nettest

This is just #cleanup to resolve a TODO

Also add a package doc.

Signed-off-by: Maisem Ali <maisem@tailscale.com>

net/memnet/memnet.go

@@ -0,0 +1,8 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package memnet implements an in-memory network implementation.
+// It is useful for dialing and listening on in-memory addresses
+// in tests and other situations where you don't want to use the
+// network.
+package memnet